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QP86  .M661  The  problem  oi  age, 


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Minot 


The  problem  of  age,  growth  and  death.  '% 


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>T, 


THE  PROBLEM  OF  AGE,  GROWTH  AND  DEATH 


CHAELES  SEDGWICK  MINOT,  LL.D.,  D.Sc. 

James  Stilbman  Pkopessoe  of  Comparative  Anatomy  in  the  Harvard 

Medical  School 


[Reprinted  from  THE  POPULAR  SCIENCE  MONTHLY,  Vol.  LXXI,  June,  August, 
September,  October,  November,  December,  1907] 


Q? 


THE 

POPULAR    SCIENCE 

MONTHLY 


JUNE,  1907 


THE   PEOBLEM    OF   AGE,   GROWTH   AND   DEATH.^ 

By  CHARLES  SEDGWICK  MINOT,  LL.D.,  D.Sc. 
JAMES  STILLMAN  PROFESSOR  OF    COMPARATIVE  ANATOMY   IN    THE  HARVARD  MEDICAL  SCHOOL. 

I.    The  Condition  of  Old  Age 

^|"^HE  subject  of  age  has  ever  been  one  which  has  attracted  human 
-*-  thought.  It  leads  us  so  near  to  the  great  mysteries  that  all 
thinkers  have  contemplated  it,  and  many  are  the  writers  who  from 
the  literary  point  of  view  have  presented  us,  sometimes  with  profound 
thought,  often  with  beautiful  images  connected  with  the  change  from 
youth  to  old  age.  We  need  but  to  think  of  two  books  familiar  more 
or  less  to  us  all — that  ancient  classic,  Cicero's  De  Senectute,  the  great 
book  on  age,  one  might  almost  say,  from  the  literary  standpoint,  and 
that  of  our  own  fellow-citizen,  my  former  teacher  and  professor  at 
the  Medical  School,  Dr.  Holmes,  who  in  his  delightful  '  Autocrat ' 
offers  to  us  some  of  his  charming  speculations  upon  age.  From  the 
time  of  Cicero  to  the  time  of  Holmes  numerous  authors  have  written 
on  old  age,  yet  among  them  all  we  shall  scarcely  find  any  one  who 
had  title  to  be  considered  as  a  scientific  writer  upon  the  subject. 
Longevity  is  indeed  a  strange  and  difficult  problem.  Many  of  you 
doubtless  have  had  your  attention  directed  recently  to  the  republished 
translation  of  Connaro's  famous  work  and  know  how  sensible  that  is, 
and  as  you  read  it  you  must  have  perceived  how  little  in  the  practical 
fispect  of  the  matter  we  have  passed  beyond  the  advice  which  old 
Connaro  gave  to  us.  And  yet  silently  in  the  medical  laboratories, 
and  in  the  physiological  and  anatomical  institutes  of  various  univer- 
eities,  we  have  been  gathering  more  accurate  information  as  to  what 
is  the  condition  of  persons  who  are  very  old. 


'  Lectures  delivered  at  the  Lowell  Institute,  Boston,  March,  1907. 


482 


POPULAR    SCIENCE   MONTHLY 


We  know,  first  of  all,  from  our  common  observation,  that  the  very 
old  grow  shorter  in  stature.  We  see  that  they  are  not  so  tall  as  in  the 
prime  of  life.  The  figures  which  have  been  compiled  upon  this  sub- 
ject are  instructive,  for  they  show  that  at  the  age  of  some  thirty  years 
the  average  height  of  men — these  figures  refer  to   Germans — is  174 


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Fig.  1.  Photograph  op  Chevreul,  taken  oa  his  one  hundredth  birthday.  He  was  asked 
to  write  in  an  album  and  replied  "Que  voulez  vous  que  j'6crive  sur  votre  album.  Je  vais 
6crire  mon  premier  priacipe  philosophique,  ce  n'est  par  moi,  qui  I'ai  formula,  c'est  Male- 
branche"On  doit  teadre  avee  effort  a  rinfallibilitS,  sans  y  priStendre."  Chevreul  was  born 
Aug.  31,  1786  and  died  Aug.  9, 1889.  For  the  privilege  of  using  this  portrait  I  am  indebted  to 
Dr.  Henry  P.  Bowditch,  to  whom  the  interesting  original  belongs. 

centimeters.  It  remains  at  that,  however,  only  for  a  short  period; 
then  it  decreases  and  at  forty  it  is  already  less;  at  fifty  decidedly  less; 
and  at  sixty  the  change  has  become  more  marked;  until  at  seventy 
years  we  find  that  the  height  has  shrunk  from  174  to  161.      There  it 


AGE,    GROWTH   AND    DEATH  483 

remains,  or  thereabouts,  through  the  remainder  of  life,  though  there 
may  be  a  small  further  diminution.  This  decrease  in  stature  is  due 
largely  to  the  changes  in  the  vertebral  column.  First  of  all  there  is 
a  stoop.  The  vertebral  column  is,  to  be  sure,  never  straight,  but  in 
old  age  it  becomes  more  curved,  and  the  result  is  a  falling  of  the  total 
stature.     But  this  is  not  the  chief  cause,  for  in  addition  to  this  the 


Fig.  2.    Photugkaph  fkom  a  Ciiii-d  at  Birth.    The  original  is  owned  by  Dr.  H.  P.  Bow - 
ditch,  by  whose  courtesy  tlie  present  reproduction  is  published. 

softer  cartilages  and  elements  of  the  spinal  column  become  harder, 
change  into  bone,  and  as  that  change  occurs  they  acquire  a  less  extent 
and  become  smaller,  and  the  result  is  that  the  vertebral  column  as  a 
whole  collapses  somewhat  and  thus  increases  the  diminution  of  height. 
We  find,  as  we  look  at  the  old,  a  great  change  to  have  come  over 
the  face.      The   roundness   of   youth   has   departed;   the  cheeks   are 


484  POPULAR    SCIENCE   MONTHLY 

sunken;  the  eyes  have  fallen  far  back;  the  lips  are  drawn  in.  All  of 
these  changes  indicate  to  us,  when  we  think  upon  them,  the  fact  that 
there  has  been  a  certain  shrinkage  and  shrivelling  of  that  which  is 
within  and  beneath  the  skin.  Expressed  in  technical  terms,  we  should 
call  this  an  atrophy,  and  to  anatomists  the  mere  sight  of  the  face  of  a 
very  old  person  reveals  at  once  this  fundamental  fact  of  an  atrophy 
of  the  parts,  an  actual  loss  of  some  of  their  bulk,  which  is  one  of  the 
most  characteristic  and  fundamental  marks  of  old  age.  The  gait 
becomes  shuffling,  the  foot  is  no  longer  lifted  free  from  the  ground, 
as  the  old  man  walks  along.  He  does  not  rise  upon  his  toes,  but  the 
sole  of  the  foot  is  kept  nearly  flat  and  as  he  drags  it  cumbrously  for- 
ward it  is  apt  to  strike  upon  the  sidewalk.  This  indicates  to  the 
physiologist  a  lessened  power  in  the  muscles,  a  lessened  control  over  the 
action  of  these  muscles,  an  inferior  coordination  of  the  movements,  so 
that  there  has  been  in  the  old  man,  judged  by  his  gait  alone,  a  physio- 
logical deterioration  as  well  as  an  anatomical  atrophy.  You  notice 
too  his  slow  speech,  often  difficult  hearing,  and  imperfect  sight.  All 
of  these  qualities  show  a  loss,  and  we  commonly  think  of  the  old  as 
those  who  have  lost  most,  who  have  passed  beyond  the  maximum  of 
development  and  are  now  upon  the  path  of  decline,  going  down  ever 
more  rapidly.  One  of  the  chief  objects  at  which  I  shall  aim  in  this 
course  of  lectures  will  be  to  explain  to  you  that  that  notion  is  erroneous, 
and  that  the  period  of  old  age,  so  far  from  being  the  period  of  true 
decline,  is  in  reality  essentially  the  period  in  which  the  actual  decline 
going  on  in  each  of  us  will  be  least.  Old  age  is  the  period  of  slowest 
decline — a  strange,  paradoxical  statement,  but  one  which  I  hope  to 
justify  fully  by  the  facts  I  shall  present  to  you  in  this  course.  In 
the  old  person  you  note  that  there  is  in  the  mind  some  failure  and 
also  loss  of  memory — less  mental  activity,  greater  difficulty  in  grasp- 
ing new  thoughts,  assimilating  new  ideas,  and  in  adapting  himself  to 
unaccustomed  situations.  All  this  betokens  again  the  characteristic 
loss  of  the  old.  And  as  we  turn  now  from  these  outward  investiga- 
tions to  those  which  the  anatomist  opens  up  to  us,  we  learn  that  in 
the  interior  of  the  body,  and  in  every  organ  thereof,  the  species  of 
change  which  I  have  referred  to  as  characteristic  of  the  very  old,  is 
going  on  and  has  become  in  each  part  well  marked.  Let  us  first 
examine  the  skeleton.  In  youth  many  parts  of  the  skeleton  are  soft 
and  flexible,  like  the  gristles  and  cartilages,  which  join  the  ribs  to 
the  breastbone,  but  in  the  old  man  they  are  replaced  by  bone. 
Bone  represents  an  advance  in  organization,  in  structure,  as  we  say, 
over  the  cartilage.  The  old  man  has  in  that  respect  progressed  be- 
yond the  youthful  stage;  but  that  progress  represents  not  a  favorable 
change;  the  alteration  in  structure  from  elastic  cartilage  to  rigid 
bone  is  physiologically  disadvantageous,  so  that  though  the  man  has 
progressed  in  the  organization  or  anatomy  of  his  body,  he  has  really 


AGE,    GROWTH    AND    DEATH  485 

thereby  rather  lost  than  gained  ground.  Indeed  in  the  skeleton  this 
principle  of  loss  is  already  revealing  itself.  In  the  interior  of  the 
bones  of  the  arms,  of  the  legs,  we  find  a  spongy  structure,  bits  of  bone 
bound  together  in  many  different  directions,  as  are  the  spicules  or 
fibers  in  a  sponge,  and  by  being  bound  so  together  they  unite  lightness 
with  strength.  As  you  know  a  column  of  metal,  if  hollow,  is  stronger 
than  the  same  amount  of  metal  in  the  form  of  a  rod.  So  with  the 
bones.  If  they  have  this  spongy  structure,  if  their  interiors  are  full 
of  little  cavities  with  intervening  spicules  acting  as  braces  in  every 
direction,  then  they  acquire  great  strength  with  little  material.  ISTow 
in  the  old  the  internal  spongy  structure  is  dissolved  away  and  there  is 
left  only  a  hard  external  shell.  Partly  on  this  condition  depends  the 
greater  liability  of  the  bones  in  the  old  person  to  break.  If  we  ex- 
amine the  muscles  we  see  that  they  have  become  less  in  volume,  and 
when  we  apply  the  microscope  to  them  we  see  that  the  single  fibers  on 
which  the  strength  of  the  muscles  depends  have  become  smaller  in 
size  and  fewer  in  number.^  The  muscle  has  actually  lost;  it  is  in- 
ferior, physiologically  speaking,  to  what  it  was  before.  You  remember 
how  melancholy  Jacques  reminded  us  of  this  fact  in  speaking  of  the 
hose  '  a  world  too  wide  for  his  shrunk  shank.'  His  saying  is  justified 
by  the  loss  of  the  muscles  in  volume  and  strength.  The  same  phe- 
nomenon of  atrophy  shows  itself  in  the  digestive  organs.  Those 
minute  structures  in  the  wall  of  the  stomach  by  which  the  digestive 
juice  is  produced,  undergo  a  partial  atrophy,  in  consequence  of  which 
they  are  less  able  to  act;  they  are  not  so  well  organized,  therefore,  not 
so  efficient  as  in  earlier  stages.  The  lungs  become  stiffened;  the 
walls  which  divide  off  an  air  cavity  from  the  neighboring  air  cavitiee 
do  not  remain  so  thin  as  in  youth,  but  become  thickened  and  hardened^ 
and  the  vital  capacity  of  the  lungs,  that  is  to  say  the  capacity  of  the 
lungs  to  take  in  and  hold  air,  is  by  so  much  lessened.  The  heart — 
it  seems  curious  at  first — is  in  the  old  always  enlarged;  but  this  does 
not  represent  a  gain  in  real  power.  On  the  contrary,  if  we  study 
carefully  the  condition  of  the  circulation  of  the  blood  in  the  old,  we 
find  that  the  walls  of  the  large  blood-vessels,  which  carry  the  blood 
from  the  heart  and  distribute  it  over  all  parts  of  the  body — vesseU 
which  we  call  arteries— have  lost  the  elastic  quality  which  is  proper 
to  them  and  by  which  they  respond  favorably  to  the  pumping  action 
of  the  heart.  Instead  they  have  become  hard  and  stiff.  We  call  this 
by  a  Greek  term  for  hardening,  sclerosis,  and  arterial  sclerosis  is  one 
of  the  most  marked  and  striking  characteristics  of  old  persons.  Now 
when  the  arteries  become  thus  stiffened,  it  requires  a  greater  force 


^  This  statement  is  the  one  currently  accepted — ^but  I  have  found,  as  yet,  no 
exact  investigation  upon  the  relative  size  and  number  of  the  muscle  fibers  in  ol^ 
persons. 


Age 
0-1 

Mean 
Frequency 
134 

1-2 

Ill 

2-3 

108 

3-4 

108 

4-5 

103 

5-6 

98 

6-  7  ...  . 

93 

7-  8  ... . 

94 

8-  9 

89 

9-10    . 

91 

10-11  .  .  .  . 

87 

11-12  

89 

12-13  

88 

486  POPULAR   SCIENCE   MONTHLY 

and  greater  effort  of  the  heart  to  drive  the  blood  through  them,  and 
in  response  to  this  new  necessity,  the  heart  becomes  enlarged  in  an 
effort  of  the  organism  to  adapt  itself  to  the  new  unfavorable  condition 
of  the  circulation  established  by  age.  But  the  power  of  the  heart  be- 
comes inferior  along  with  this  hypertrophy  or  enlargement,  and  we  see 
that  in  the  old,  in  order  to  make  up  for  the  feebleness  of  the  enlarged 
heart,  it  beats  more  frequently.  In  other  words,  the  pulse  rate  in 
the  old  person  increases.^     We  find,  for  instance,  that  at  the  time  of 

Mean  Me>in 

Age                         Preqiienoy  ^"^  Freqnenoy 

13-14 87  25-30    72 

14-15 82  30-35    70 

15-16 83  35-40    72 

16-17 80  40-45    72 

17-18 76  45-50    72 

18-19 77  50-55    72 

19-20 74  55-60    75 

20-21 71  60-65    73 

21-22 71  65-70    75 

22-23 70  70-75    75 

23-24 71  75-80    72 

24-25 72  80  and  over    79 


birth  the  pulse  rate  is  at  the  rate  of  13i  beats  to  a  minute.  It  rises 
slightly  during  the  first  three  months  of  infancy  until  at  the  end  of 
the  third  month  it  reaches  some  140  beats  a  minute;  it  soon  falls  off, 
however,  and  at  the  end  of  the  first  3^ear  it  has  sunk  to  111;  at  five 
or  six  years  it  becomes  98,  and  at  twenty-one  years  it  has  sunk  to  71 
or  72.  There  are  thereafter  certain  minor  fluctuations  in  the  rate 
of  the  heart-beat  Avith  advancing  age,  but  generally  it  may  be  said  that 
this  value  of  72  beats  a  minute  is  characteristic  of  adult  life.  But 
when  a  person  becomes  eighty  years  old,  it  has  been  found  that  upon 
the  average  the  rate  of  the  heart-beat  rises  and  becomes  79  a  minute. 
Hence  it  is  clear  that  though  the  heart  is  larger,  it  has  to  make  a 
greater  effort,  that  is  to  say  a  more  frequent  beat,  in  order  to  main- 
tain the  necessary  circulation  of  the  blood.  We  see  also,  as  we  go  back 
to  the  anatomical  examination  of  the  body,  that  those  important 
jitructures  which  we  call  the  germ  cells,  upon  which  the  propagation 
of  the  race  depends,  which  present  under  the  microscope  certain  clearly 
recognized  characteristics  by  which  they  can  be  distinguished  from  all 
other  cells  of  the  body,  that  these  germ-cells  cease  their  activity  alto- 
gether in  the  very  old,  and  one  of  the  great  functions  of  life  is  thus 
blotted  out  altogether  from  the  history  of  the  individual. 

Turning  now  to  the  yet  nobler  organs,  especially  the  brain,  we  see 


'My  friend,  Professor  W.  T.  Porter,  has  had  the  kindness  to  compile  the 
following  table  for  me,  showing  the  pulse  frequency  from  one  to  eighty  years. 
For  the  first  two  months  after  birth,  the  rate  is  about  130,  after  the  third  month 
140.    The  foetal  rate  is  135  to  140. 


AGE,    GROWTH   AND    DEATH  487 

a  curious  change  going  on,  a  change  of  which  old  age  presents  to  us 
the  culminating  record.  In  order  to  study  the  weight  of  the  brain, 
it  is  necessary  to  compare  people  of  the  same  size,  for  the  size  and 
weight  of  the  brain  depend  somewhat  upon  the  size  of  the  individual. 
Now  it  has  been  discovered  by  careful  examination  of  persons  of 
similar  size  that  the  brain  begins  relatively  early  to  diminish  its 
weight.  Thus  in  persons  of  a  height  of  175  centimeters,  and  over, 
of  the  male  sex,  it  is  found  that  in  a  period  of  from  twenty  to  forty 
years  the  brain  weight  is  1,409  grams.  But  from  forty-one  to  seventy 
years  it  has  sunk  to  1,363,  and  in  persons  of  from  seventy-one  to 
ninety  it"  has  shrunk  to  1,330.  Women  of  corresponding  size  are  not 
easily  found,  and  a  more  average  height  for  women  is  165  centimeters; 
a  woman  of  such  a  height  is  likely  to  have — among  the  white  races, 
be  it  always  understood — a  weight  of  brain  of  1,265  grams,  at  forty 
to  seventy  years  a  brain  of  1,200,  and  at  seventy-one  to  ninety  years 
a  brain  of  only  1,166  grams.*  I  give  these  figures  because  they  show 
that  there  is  no  guessing,  but  a  definite,  positive  knowledge,  proving 
that  soon  after  the  maturity  of  life  in  the  individual  is  reached,  the 
shrinkage  of  the  brain  begins,  and  then  continues  almost  steadily  to 
the  very  end  of  life. 

It  is  not  only  the  anatomist,  but  it  is  perhaps  almost  equally  the 
physiologist  who  gives  us  insight  into  the  changes,  which  go  on  in 
the  old.  I  spoke  a  few  moments  ago  of  the  pulse  rate,  and  of  the 
change  which  that  offers.  At  first  sight  it  seems  as  if  a  greater  pulse 
rate  indicated  an  improvement,  but  if  you  recall  the  explanation  which 
I  have  given  you,  you  will  acknowledge  that  this  is  by  no  means  an 
acceptable  interpretation,  but  that  on  the  contrary  the  change  is  a 
clear  mark  of  enfeeblement.  In  the  respiration,  also,  we  observe  a 
like  change.  Here  the  comparison  is  not  quite  so  easy  as  we  should 
at  first  imagine,  because  there  is  a  relation  between  the  size  of  the 
individual  and  the  respiration.  The  respiration,  as  you  all  know,  frees 
the  body  from  the  products  of  combustion,  particularly  from  that 
product  which  we  know  as  carbon  dioxide.  The  result  of  the  com- 
bustion going  on  in  the  body  (which  in  its  end  term  appears  to  us  as 
carbon  dioxide  expelled  from  the  lungs)  is  to  produce  heat,  to  de- 
velop the  necessary  warmth  for  the  maintenance  of  the  proper  tem- 


*  Ernst  Handmann  has  recently  published  statistics  on  the  growth  of  brain, 
based  on  measurements  at  the  Leipzig  Pathological  Institute.  See  Archiv  f. 
Anat.  u.  Entwickelungsges.,  1906,  p.  1.     The  following  summarizes  his  results: 

Brain  Weight  in  Grama 
Age  Male  Female 

4-6    1215  1194 

7-14    1376  1229 

15-49    1.372  1249 

50-84    (89)    1332  1196 


488  POPULAR    SCIENCE   MONTHLY 

perature  of  the  body.  Now  in  the  very  young  the  bulk  of  the  body 
is  not  great,  but  the  loss  of  heat  is  very  great,  and  this  perhaps  can  be 
most  readily  explained  to  you  if  you  imagine  that  you  hold  in  one 
hand  a  very  small  potato  and  in  the  other  a  very  large  potato,  both  of 
which  have  come  at  the  same  moment  from  the  same  oven,  and  that 
you  have  just  started  out  for  a  cold  winter  drive.  You  all  know,  of 
course,  that  in  a  little  while  the  small  potato,  though  it  was  as  hot  as 
the  large  one  at  first,  will  have  lost  its  heat,  will  no  longer  serve  to  keep 
the  hand  warm,  but  the  other  hand,  in  which  the  bulkier  potato  is  held, 
in  which  the  volume  of  the  heat — we  might  so  express  it,  perhaps — is 
correspondingly  great,  benefits  by  the  retained  heat  a  long  time.  Es- 
sentially similar  to  this  is  the  difference  between  the  child  and  the 
adult.  The  child  loses  heat  with  comparatively  great  rapidity — ^the 
old  person  at  a  comparatively  slow  rate.  Hence  it  is  necessary  for 
the  child  to  produce  more  warmth  in  order  to  keep  up  the  natural 
normal  temperature  of  the  body.  When,  therefore,  we  find  that  in  the 
old  person  the  respiration  is  diminished,  and. that  the  production  of 
carbon  dioxide  from  the  lungs  is  greatly  lessened,  we  are  not  immedi- 
ately to  jump  at  the  conclusion  that  the  quality  of  physiological  action 
has  been  debased — that  we  see  here  a  sign  of  decrepitude.  On  the 
contrary,  the  change  is  the  result  of  physiological  adaptation,  of  suit- 
ing the  performance  of  the  body  to  its  needs.  This  is  one  of  the  great 
wonders,  one  of  the  mysteries  of  life,  of  which  we  here  have  a  sample, 
the  constant  adaptation  of  the  means  to  the  end.  That  which  the 
body  needs  is  done  by  the  body.  A  child  needs  more  warmth,  and 
its  body  produces  more ;  the  old  person  needs  less  warmth,  and  his  body 
produces  less.  How  this  is  accomplished  we  are  unable  to  say,  but 
constantly  we  see  evidence  of  this  purposeful  accommodation  on  the 
part  of  the  body — what  is  called  by  the  physiologists  the  teleological 
principle,  the  adaptation  of  the  reaction  of  the  body  to  its  needs. 
There  are  innumerable  illustrations  of  this,  many  of  which  are  of 
course  perfectly  familiar  to  us,  although  perhaps  we  do  not  think  of 
them  as  illustrations  of  this  great  law  of  nature.  As,  for  instance, 
when  we  eat  a  meal,  and  the  presence  of  food  in  the  stomach  calls  into 
action  the  glands  in  the  wall  of  the  stomach  by  which  the  digestive 
juice  is  secreted.  The  juice  is  produced  exactly  at  the  time  when  it  is 
needed.  Innmuerable,  indeed,  are  the  illustrations  of  this  fundamental 
principle. 

There  is  another  class  of  phenomena  characteristic  of  the  very  old 
which  will  perhaps  seem  a  little  surprising  to  you  after  the  general 
tenor  of  my  previous  remarks.  I  refer  to  the  power  of  repair.  This, 
modern  surgery  especially  has  enabled  us  to  recognize  as  being  far 
greater  in  the  old  than  we  were  wont  to  assume;  and  we  know  that 
there  is  a  certain  luxury,  a  certain  excess  reserve  in  the  power  of  re- 
pair, and  that  we  may  go  far  beyond  the  ordinary  necessities  of  our 


AGE,    GROWTH    AND    DEATH  489 

life  in  our  demands  upon  our  organism,  and  still  find  that  our  body 
is  capable  of  making  the  necessary  response.      Ordinarily  the  amount 
of  blood  which  we  require  is  moderate  in  amount — moderate  in  the 
sense  that  the  destruction  of  the  blood  continually  going  on  in  the 
body  is  not  a  very  rapid  process ;  but  if,  through  some  accident,  a  person 
loses  a  large  quantity  of  blood  then  by  one  of  these  teleological  reac- 
tions of  which  I  have  spoken,  the  production  of  new  blood  is  increased, 
the  loss  is  soon  made  up,  and  we  discover  that  the  blood,  so  to  speak, 
has  been  repaired.     Or  when  a  little  of  the  skin  is  lost,  it  quickly  heals 
over.     That  again  is  due  to  the  power  of  repair.      Ordinarily  so  long 
as  the  skin  remains  whole  that  power  is  not  called  into  action,  but 
if  a  wound  comes,  then  the  regenerative  force  resident  always  in  the 
skin,  but  inactive,  comes  into  play  and  produces  the  mending  which 
is  such  a  comfort.      So  in  old  people,  some  of  this  luxury  of  reparative 
power  persists,  so  that  they  can  recover  from  wounds  in  a  far  better 
way  than  we  should  imagine  if  we  judged  them  only  by  the  general 
physiological   and   anatomical   decline   exhibited  throughout   all  parts 
of  the  body.     Some  of  the  luxury  of  repair  comes  in  usefully  in  old  age. 
Now  if  we  consider  all  these  changes  in  the  most  general  manner, 
we  perceive  that  they  are  clearly  of  one  general  character;  they  imply 
an  alteration  in  the  anatomical  condition  of  the  parts;  but  it  is  an  al- 
teration which  does  not  differ  fundamentally  in  kind  from  the  alterations 
which  have  gone  on  before,  but  it  does  differ  in  the  extent  and  in  part 
in  the  degree  to  which  these  alterations  have  taken  place.      When  the 
elastic  cartilaginous  rib  becomes  bony,  nothing  different  is  happening 
from  that  which  happened  before,  for  there  was  a  stage  of  development 
when  the  entire  rib  consisted  of  cartilage,   and  in  the  progress   of 
development  toward  the  adult  condition  that  cartilage   was  changed 
gradually  into  bone,   thus  producing  the  characteristic,   normal,   effi- 
cient bony  rib  of  the  adult.     When  old  age  intervenes,  the  change  of 
the  cartilage  into  bone  goes  yet  further,  but  it  progresses  in  such  a  way 
that  it  is  no  longer  favorable,  but  unfavorable.     We  have  then  in  this 
case  a  clear  illustration  of  a  principle  of  change  in  the  very  old  which 
is,  I  take  it,  perhaps  sufficiently  well  expressed  by  saying  that  the 
change  which  is  natural  in  the  younger  stage  is  in  the  old  carried  to 
excess.      But  there  is  in  addition  to  this,  something  more,  of  which 
I  have  already  spoken,  namely  the  atrophy  of  parts,  and  by  atrophy 
we  mean   the  diminution,  the  lessening  of  the  volume  of  the  part. 
There  is  a  partial  atrophy  of  the  brain  in  consequence  of  which  that 
organ  becomes  smaller;  there  is  an  extensive  atrophy  of  the  muscles 
in  consequence  of  which  their  volume  is  diminished,  and  their  efficiency 
decreased.      Atrophy  is  preeminently  characteristic  of  the  very  old, 
and  we  see  in  very  old  persons  that  it  becomes  each  year  more  and 
more   pronounced.     Indeed,   it  has   l)een   said   recently   by   Professor 
Metchnikoff,  a  distinguished  Russian  zoologist,  now  connected  with  the 


490'  POPULAR    SCIENCE   MONTHLY 

Pasteur  Institute  in  Paris,  some  of  whose  publications  many  of  you 
have  doubtless  read,  that  his  conception  of  the  nature  of  senility,  of 
old  age,  could  best  be  expressed  in  a  single  word,  atrophy.  "  On  resume 
la  senilite  par  un  seul  mot:  atrophie."^  That  is  his  estimate  of  old 
age.  But  that  is  not  the  only  estimate  of  old  age  which  has  been  made 
up  to  the  present  time.  We  find  one,  which  is  much  more  prevalent, 
is  that  which  connects  it  with  the  condition  of  the  arteries.  Indeed, 
Professor  Osier  has  written  this  sentence — "Longevity  is  a  vascular 
question,  and  has  been  well  expressed  in  the  axiom  that  a  man  is  only 
as  old  as  his  arteries."  Now  these  are  medical  views,  not  biological, 
and  you  will  find  that  there  is  a  very  extensive  literature  dealing 
with  old  age  in  man  based  upon  the  conception  that  old  age  is  a  kind 
of  disease,  a  chronic  disease,  an  incurable  disease.  Medical  writers 
have  put  forward  various  conceptions  giving  a  medical  interpretation 
of  this  disease.  That  to  which  I  just  referred  is  the  favorite  one,  the 
one  you  are  most  likely  to  hear  from  physicians  to-day — namely,  the 
theory  of  arterial  sclerosis,  that  the  hardening  of  the  walls  of  the 
arteries  is  the  primary  thing;  it  interferes  with  the  circulation,  the 
bad  circulation  interferes  with  the  proper  working  of  every  part  of  the 
body,  and  as  the  circulation  becomes  impeded,  various  accessory  results 
are  produced  in  the  body  in  consequence.  It  is  brought  to  a  lower 
or  more  diseased  condition  than  before.  And  so  they  interpret  sclerosis 
of  the  arteries  as  the  primary  thing,  because  they  can  trace  so  many 
alterations  in  the  old  which  resemble  diseased  alterations,  to  these 
natural  changes  in  the  arteries  by  which  they  acquire  hardened  and 
inelastic  walls,  which  prevent  the  proper  response  of  the  artery  to 
the  heart  beat,  upon  which  the  normal  healthy  circulation  largely 
depends.  Another  interpretation,  very  curious  and  interesting,  is  that 
which  has  been  recently  offered  by  the  same  Professor  Metchnikoff . 
whom  I  have  just  mentioned.  He  has  written  a  book  upon  the  '  Nature 
of  Man,'  translated  in  1903,  and  published  in  this  country.  It  is  an 
interesting  book.  It  gives  a  most  attractive  picture,  incidentally,  of 
Metchnikoff  himself,  a  man  of  pleasantly  optimistic  temperament,  but 
a  man  thoroughly  imbued  with  the  spirit  which  has  so  often  been 
attributed  to  contemporary  scientific  men^  of  cold,  intellectual  regard 
towards  everything,  towards  life,  towards  man,  towards  mystery.  For 
him  mysteries  of  all  sorts  have  little  interest.  Those  things  which 
are  mysterious  are  beyond  the  sphere  of  what  can  hold  his  attention. 
He  must  reside  in  the  clear  atmosphere  of  definite,  positive  fact.  This 
mental  bias  is  shown  in  his  book.  He  reviews  in  a  happy  way  various 
past  systems  of  philosophy;  he  describes  various  religions;  and  he 
points  out  his  reasons  for  thinking  that  all  of  these  are  insufficient, 
that  there  is  no  satisfaction  to  be  derived  from  any  of  the  ancient 

^UAnnee  biologique,  Tome  III.,  p.  256,  1897. 


AGE,    GROWTH    AND    DEATH  491 

philosophies  or  from  any  of  the  great  world  religions.  Nevertheless 
he  is  an  optimist.  He  has  noticed  as  a  result  of  his  meditations  upon 
the  arrangements  within  our  bodies  that  we  suffer  very  much  from 
what  he  calls  disharmonies,  by  which  he  means  imperfect  adaptations 
of  structures  within  us  to  the  performance  of  the  body  as  a  whole. 
He  mentions  various  instances  of  such  disharmonious  parts.  They 
do  not  seem  to  me  quite  so  imposing  as  apparently  they  do  to  him, 
for  many  of  his  disharmonies  are  based  upon  the  fact  that  we  do  not 
know  that  a  certain  structure  or  part  has  any  useful  role  to  play  in 
the  body.  But  I  am  inclined  to  suspect  that  in  many  cases  it  is  only 
because  we  are  ignorant;  the  list  of  useless  structures  in  the  human 
body  was  a  few  years  ago  very  long;  it  has  within  recent  years  been 
greatly  shortened,  and  we  should  learn  from  this  experience  a  caution 
in  regard  to  judging  about  these  things,  which,  I  think.  Professor 
Metchnikoff  has  failed  to  exert  duly  in  forming  his  opinions  on  these 
disharmonies.  Xow  among  the  disharmonies  which  he  recognizes  is 
that  of  the  great  size  of  the  large  intestine,  which  is  of  such  a  caliber 
that  a  considerable  quantity  of  partially  digested  food  can  be  retained 
in  it  at  one  time.  When  such  food  is  retained  in  the  intestine,  it 
may  undergo  a  process  of  fermentation.  There  are  many  sorts  of 
fermentation,  and  some  of  them  produce  chemical  bodies  which  are 
injurious  to  the  human  organism.  Bacteria,  which  will  cause  fer- 
mentation of  this  sort,  do  actually  occur  in  the  human  intestine. 
Metchnikoff  thinks  that,  as  we  grow  old,  this  tendency  to  fermentation 
increases.  ISTow  the  bodies  produced  by  fermentation,  the  chemical 
bodies,  I  mean,  get  into  our  system  and  poison  us.  The  result  of  the 
poisoning  is  that  the  native  capacities  of  the  various  tissues  and  organs 
of  the  body  are  lowered,  as  happens  in  a  man  '  intoxicated.'  All  parts 
of  a  man  may  be  poisoned,  not  necessarily  always  with  alcohol,  but 
with  many  other  things  as  well,  and  such  a  poisoning  Professor 
Metchnikoff  assumes  to  result  from  intestinal  fermentation.  More- 
over, he  has  further  observations,  which  lead  him  to  the  idea  that 
certain  cells  go  to  work  upon  the  poisoned  parts  and  do  further  damage. 
The  cells  in  question  are  minute  microscopic  structures,  so  small  that 
we  can  not  at  all  see  them  with  the  naked  eye,  but  which  have  a 
habit  of  feeding  in  the  body  upon  the  various  parts  thereof  whenever 
they  get  a  chance.  Cells  of  this  sort  go  by  the  scientific  name  of 
phagocytes,  which  is  merely  a  Greek  term  for  '  eating  cells.'  The 
phagocytes,  for  instance,  devour  pigment  in  the  hair,  and  in  old  per- 
sons the  production  of  white  hair  has  resulted  from  the  activity  of 
phagocytes  which  have  eaten  the  pigment  which  should  have  remained 
in  the  hair  and  kept  its  color.  But  the  pigment  of  the  hair  is  not  the 
only  thing  they  will  attack;  they  will  make  their  aggressive  inroads 
upon  any  part  of  the  body;  and  Professor  Metchnikoff  has  advanced 
the  theory  that  old  age  consists  chiefly  in  the  damage  which  is  done 


492  POPULAR    SCIENCE   MONTHLY 

by  phagocytes  to  poisoned  parts  of  the  body,  the  poisoning  being  due 
to  the  fermentation  in  the  large  intestine.  Now  it  has  been  observed 
by  some  of  the  German  investigators  of  these  matters  that  the  presence 
of  lactic  acid  interferes  with  this  fermentative  process  as  it  goes  on  in 
the  intestine.  Lactic  acid,  as  its  name  implies,  is  the  characteristic 
acid  which  occurs  in  milk  when  it  becomes  sour.  An  Italian  friend 
of  Professor  Metehnikoff  tried  drinking  some  sour  milk  with  the  idea 
of  stopping  the  fermentation  in  the  intestine,  and  so  putting  an  end 
to  the  deleterious  change,  and  he  believes  in  the  short  time  that  he  tried 
it  that  it  did  him  good— quite,  you  see,  in  the  way  of  a  patent  medicine. 
Professor  Metchnikoff,  on  this  basis,  has  recommended,  in  his  book 
on  the  'Nature  of  Man,'  the  regular  drinking  of  sour  milk,  in  the 
hope  apparently  that  that  will  postpone  senility,  and  will  leave  us  our 
powers  in  maturity  long  beyond  that  period  when  we  at  present  reach 
the  fullness  of  our  vigor,  and  advance  the  period  of  time  when  the 
changes  of  the  years  put  us  out  of  court.  He  regards  this  as  an  opti- 
mistic substitute  for  the  various  forms  of  philosophy  and  religion 
which  many  millions  of  people  have  found  helpful  in  life,  and  cer- 
tainly it  is  the  cheapest  substitute  which  has  ever  been  seriously 
proposed. 

There  is  another  writer  who,  though  having  a  German  name,  is  in 
reality  a  Eussian,  Professor  Miihlmann.  He  has  another  theory  in 
regard  to  the  fundamental  nature  of  senility.  He  takes  such  in- 
stances as  that  which  I  spoke  of,  of  respiration  in  connection  with 
the  production  of  warmth  in  the  child's  body  and  in  the  body  of  the 
adult,  and  finds  that  the  diminution  of  the  surface  in  proportion  to 
the  bulk  of  the  body  is  characteristic  of  the  old,  and  he  concludes 
that  we  become  old  because  we  do  not  have  proportionately  surface 
enough  left.  His  view  implies,  apparently,  that  if  we  could  keep 
ourselves  more  or  less  of  the  stature  of  pygmies  we  should  be  healthier 
and  better  off.  I  confess  these  theories,  and  many  others  which  I 
might  enumerate  to  you,  seem  to  me  to  be  somewhat  fantastic — odd 
rather  than  valuable.  Yet  they  all  spring  from  this  one  common 
feeling,  which  is,  I  believe,  a  sinister  influence  upon  the  thought  of 
the  day,  in  regard  to  the  problem  of  age — they  spring  from  the  medi- 
cal conception  that  age  is  a  kind  of  disease,  and  that  the  problem  is 
to  explain  the  condition  as  it  exists  in  man.  Now  that  is  precisely  what 
I  wish  to  protest  against.  What  I  hope  to  accomplish  in  these  lec- 
tures is  to  build  up  gradually  in  your  minds  some  acquaintance  with 
the  fundamental  and  essential  changes,  which  are  characteristic  of 
age  and  in  regard  to  which  we  have  been  learning  something  during 
the  last  few  years — I  might  almost  say  only  within  recent  years — 
and  by  means  of  this  exposition  to  give  you  a  broader  view  and  a  juster 
interpretation  of  the  problem.  I  hope,  before  I  finish,  to  convince 
you  that  we  are  already  able  to  establish  certain  significant  generaliza- 


AGE,    GROWTH    AND    DEATH  493 

tions  as  to  what  is  essential  in  the  change  from  youth  to  old  age,  and 
that  in  consequence  of  these  generalizations,  now  possible  to  us,  new 
problems  present  themselves  to  our  minds,  which  we  hope  really  to  be 
able  to  solve,  and  that  in  the  solving  of  them  we  shall  gain  a  sort  of 
knowledge,  which  is  likely  to  be  not  only  highly  interesting  to  the 
scientific  biologist,  but  also  to  prove,  in  the  end,  of  great  practical 
value.  Surely  we  can  not  hope  to  obtain  any  power  over  age,  any 
power  over  the  changes  which  the  years  bring  to  each  of  us,  unless  we 
understand  clearly,  positively  and  certainly,  what  these  changes  really 
are.  I  think  you  will  learn,  if  you  do  me  the  honor  to  follow  the 
lectures  further,  that  the  changes  are  indeed  very  different  from  what 
we  should  expect  when  we  start  out  on  a  study  of  age,  and  that  the 
contributions  of  science  in  this  direction  are  novel  and  to  some  degree 
startling.  We  can  begin  to  approach  this  broader  view  of  our  subject 
if  we  pass  beyond  the  consideration  of  man. 

If  we  turn  from  man  to  the  animals  which  we  are  most  familiar 
with,  the  common  domestic  quadrupeds,  we  see  that  they  undergo  a 
series  of  changes  not  very  dissimilar  to  those  which  man  himself  must 
pass  through.  An  old  horse,  an  old  dog,  an  old  cat,  shows  pretty  much 
the  same  sort  of  decrepitudes  which  characterize  old  men.  But  when 
we  pass  farther  down  in  the  scale  to  the  fishes,  or  even  to  a  frog,  we  dis- 
cover great  differences.  Do  you  think  you  could  tell  a  frog  when 
it  is  old  by  the  way  it  walks — for  it  never  walks — or  a  fish  by  the 
amount  of  hardening  of  the  lungs,  when  it  has  none?  Yet  the  lack 
of  lungs  is  characteristic  of  the  fish.  And  what  becomes  of  the  theory 
of  arterial  sclerosis  when  we  go  still  lower  in  the  animal  kingdom, 
towards  its  lowermost  members,  and  find  creatures  which  live  and 
thrive  and  have  lived  and  thriven  for  countless  generations,  yet  have 
no  arteries  at  all?  They,  of  course,  do  not  grow  old  by  any  change 
of  their  arteries.  But  when  we  come  to  study  these  various  animals 
more  carefully,  we  learn  that  in  them  the  anatomical  and  physiological 
features  which  I  have  indicated  to  you  in  my  description  of  the  changes 
in  the  human  being,  are  paralleled,  as  it  were,  by  similar  changes; 
but  only  by  similar,  not  by  identical,  changes.  If  we  examine  the 
insects,  for  instance,  we  see  that  in  an  old  insect  there  is  a  hardening 
of  the  outer  crust  of  the  body  which  serves  as  a  shell  and  a  skeleton 
at  once.  That  hardening  increases  with  the  age  of  the  individual. 
We  can  see  in  the  insect  a  lessening  development  of  the  digestive  tract, 
and  we  can  see — it  has  been  demonstrated  with  particular  nicety — a 
degradation  of  the  brain.  Insects  have  a  very  small  brain,  but  wlien 
a  bumblebee,  or  a  honeybee,  grows  old,  as  he  does  in  a  few  weeks  after 
he  acquires  his  wings,  we  see  that  the  brain  actually  becomes  smaller, 
and  not  only  that,  but  as  I  shall  be  able  to  demonstrate  to  you  with 
the  lantern  in  the  next  lecture,  the  elements  which  build  up  the  brain 
have  each  of  them  become  smaller  and  the  diminution  in  the  size  of 


494  POPULAR    SCIENCE   MONTHLY 

the  brain  is  due  in  part  to  the  shrinkage  of  the  single  microscopic  con- 
stituents. There  is  another  point  of  resemblance.  We  find  that  when 
one  of  the  better  parts  of  the  body  undergoes  an  atrophy,  it  becomes 
not  only  smaller,  but  its  place  is  to  a  certain  extent  taken  by  the  in- 
ferior tissues — especially  by  those  which  we  call  comprehensively  the 
connective  tissues,  which  might  perhaps  be  best  described  to  a  general 
audience  as  that  which  is  the  stuffing  of  the  body  and  fills  out  all  the 
gaps  between  the  organs  proper.  In  consequence  of  performing  this 
general  function,  they  are  very  properly  called  connective  tissues, 
since  they  connect  all  the  different  organs  and  systems  of  organs  in 
the  body  together.  Now  in  every  body  there  is  a  continual  fighting 
of  the  parts.  They  battle  together,  they  struggle,  each  one  to  get 
ahead,  but  the  nobler  organ,  generally  speaking,  holds  its  own.  There 
are  early  produced  from  the  brain  the  fine  bundles  of  fibers  which  we 
call  the  nerves,  which  run  to  the  nose,  to  the  tongue  and  to  the  various 
parts  of  the  body.  When  these  appear  all  the  parts  of  the  body  are 
very  soft.  Afterwards  comes  in  the  hard,  and,  we  should  think,  sturdy 
bone,  but  never,  under  normal  conditions,  does  the  bone  grow  where 
the  nerve  is.  The  nerve,  soft  and  pulpy  as  it  seems,  resists  absolutely 
the  encroachment  of  the  bone,  and  though  the  bone  may  grow  else- 
where, and  will  grow  elsewhere  the  moment  it  gets  a  free  opportunity, 
it  can  not  beat  the  soft  delicate  nerve.®  Similarly  we  find  that  the 
substance  which  forms  the  liver  is  pulpy,  very  delicate.  Those  of 
you  who  have  seen  fresh  liver  in  the  butcher's  shop  know  what  a  flabby 
organ  it  is,  and  yet  though  it  is  surrounded  by  the  elements  of  con- 
nective tissue,  which  with  great  zest  and  eagerness  produce  tough 
fibers,  it  never  gives  way  to  them.  The  connective  tissue  is  held 
back  by  the  soft  liver  and  kept  in  place  by  it.  The  liver  is,  so  to  speak, 
a  nobler  organ  than  the  connective  tissue  and  holds  sway  ordinarily; 
but  in  old  age,  when  the  nobler  organs  lose  something  of  their  power, 
then  the  connective  tissue  gets  its  chance,  grows  forward  and  fills 
up  the  desired  place,  and  acquires  more  and  more  a  dominating  posi- 
tion. We  can  see  this  alike  in  the  brain  of  man  and  in  the  brain  of 
the  bee.  That  which  is  the  nervous  material  proper,  microscopic  ex- 
amination shows  us  to  be  diminished  everywhere  in  the  old  bee  and  in 
the  old  man,  and  the  tissue  which  supports  it,  which  is  of  a  coarser 
nature  and  can  not  perform  any  of  the  nobler  functions,  fills  up  all 
the  space  thus  left,  so  that  the  actual  composition  of  the  brain  is  by 
this  means  changed.     There  is,  you  see,  therefore,  during  the  atrophy 

'  The  nerve  fibers  of  the  olfactory  membrane  arise  very  early  in  the  embryo 
and  form  numerous  separate  bundles.  Later  the  bone  arises  between  the  bundles, 
for  each  of  which  a  hole  is  left  in  the  osseous  tissue,  so  that  the  bone  in  the 
adult  has  a  sieve-like  structure,  and  hence  is  termed  the  cribriform  plate.  It 
offers  a  striking  illustration  of  the  inability  of  hard  bone  to  disturb  soft 
nerve  fibers. 


AGE,    GROWTH   AND    DEATH  495 

of  the  brain,  not  only  a  diminution  of  the  organ  as  a  whole,  but  there 
is  the  further  degradation  which  consists  in  the  yielding  of  the  nobler 
to  the  baser  part,  if  I  may  so  express  myself.  That,  you  recognize, 
necessarily  implies  a  loss  of  function.  The  brain  can  not  under  senile 
conditions  do  the  sort  of  fine  and  efficient  work  which  it  could  do  before. 
Now  if  we  go  on  from  insects  to  yet  lower  organisms,  we  see  less  and 
less  appearing  of  an  advance  in  organization,  of  correlated  loss  of  parts, 
and  when  we  get  far  enough  down  in  the  scale,  senescence  becomes 
very  vague.  The  change  from  youth  to  old  age  in  a  coral  or  in  a 
sponge  is  at  best  an  indefinite  matter. 

I  should  like,   did  the  length  of  the   course  permit,   to   enlarge 
greatly  upon  this  aspect  of  the  question,  and  explain  to  you  how  it 
is  that  as  the  organism  rises  higher  and  higher  in  the  scale,  old  age 
becomes  more  and  more  marked,  and  in  no  animal  is  old  age  perhaps 
so  marked,  certainly  in  no  animal  is  it  more  marked,  than  in  ourselves. 
The  human  species  stands  at  the  top  of  the  scale  and  it  also  suffers 
most  from  old  age.      We  shall  learn,  I  hope,  more  clearly  later  on  in 
the  course  of  these  lectures,  that  this  fact  has  a  deeper  significance, 
that  the  connection  between  old  age  and  advance  in  organization,  ad- 
vance in  anatomical  structure,  is  indeed  very  close,  and  that  they  are 
related  to  one  another  somewhat  in  fashion  of  cause  and  effect;  just 
how  far  each  is  a  cause  and  how  far  each  is  an  effect  it  would  perhaps 
be  premature  to  state  very  positively;  but  I  shall  show  you,  I  think  in 
a  convincing  way,  that  the  development  of  the  anatomical  quality, 
or  in  other  words  of  what  we  call  organic  structure,  is  the  fundamental 
thing  in  the  investigation  of  the  processes  of  life  in  relation  to  age. 
We  can  see  it  illustrated  again  very  clearly  indeed  when  we  turn  to  the 
study  of  plant  life,  for  plants  also  grow  old.      Take  a  leaf  in  the 
spring.     It  is  soft  as  the  bud  opens.     The  young  leaf  is  delicate.      It 
has  a  considerable  power  of  growth.      It  expands  freely,  and  soon 
becomes  a  leaf  of  full  size.     Then  comes  the  further  change  by  which 
the  leaf  gets  a  firmer  texture;  the  production  of  anatomical  quality 
in  the  leaf,  so  to  speak,  goes  on  through  the  summer,  and  the  result 
of  that  advance  in  the  anatomical  quality  is  that  the  delicate,  youthful 
softness  and  activity  of  the  leaf  is  stopped.      It  can  not  grow  any 
more;  it  can  not  function  as  a  leaf  properly  any  more.     The  develop- 
ment of  its  structure  has  gone  too  far  and  the  leaf  falls  and  is  lost, 
and  must  be  replaced  by  a  new  leaf  the  next  year.     When  we  examine 
the  changes  that  go  on  in  any  flowering  plant,  we  observe  always  that 
there  is  this  production  of  structure,  and  then  the  decay,  the  end  or 
death.      At  first  structure  comes  as  a  helpful  thing,  increasing  the 
usefulness  of  the  part,  and  then  it  goes  on  too  far  and  impairs  the 
usefulness,  and  at  last  a  stage  is  produced  in  which  no  use  is  possible 
any  longer — the  thing  is  worthless.     It  is  cast  away  in  the  case  of  the 
plant  life;  and  this  casting  away  of  the  useless  is  a  thing  not  by  any 


496  POPULAR    SCIENCE   MONTHLY 

means  confined  to  plants;  it  occurs  equally  in  ourselves  all  the  time; 
at  every  period  of  our  life  we  have  been  getting  through  with  some  por- 
tion of  our  body;  that  portion  acquired  a  certain  organization,  it 
worked  for  us  awhile,  and  then  being  done  with  it,  we  threw  it  away 
because  it  was  dead.  Very  early  in  the  history  of  every  individual 
there  was  a  production  of  blood,  and  then  followed  the  destruction  of 
some  of  the  blood  corpuscles  and  their  remains  were  used  for  various 
purposes.  The  pigment  which  is  in  the  liver  comes  from  the  destroyed 
blood  corpuscles,  and  it  is  believed  that  the  pigment  which  colors  the 
hair  is  derived  from  the  same  source.  The  blood  corpuscles  contain 
a  material  which  when  chemically  elaborated  reappears  as  the  deposit 
which  imparts  to  the  hairs  their  coloration.  You,  of  course,  are  all 
familiar  with  the  loss  of  hair.  It  occurs  to  everybody,  but  did  you 
ever  think  that  it  means  that  the  hair  which  has  lived  has  died,  and 
that  that  hair  which  was  a  part  of  you  has  been  cast  off  ?  That  is  what 
the  loss  of  hair  means  to  the  biologist — the  death  of  a  part  and  the 
throwing  away  of  it,  and  it  is  typical  of  what  is  going  on  through  the 
body  all  the  time.  It  occurs  in  the  intestines,  where  the  elements 
which  serve  for  purposes  of  digestion  are  continually  dying  and  being 
cast  off.  The  outer  skin  is  constantly  falling  off  and  being  renewed, 
and  that  which  goes  is  dead.  In  every  part  of  the  body  we  can  find 
something  which  is  dying.  Death  is  an  accompaniment  of  develop- 
ment; parts  of  us  are  passing  off  from  the  limbo  of  the  living  all  the 
time,  and  the  maintenance  of  the  life  of  each  individual  of  us  depends 
partially  upon  the  continual  death  going  on  in  minute  fragments  of 
our  body  here  and  there. 

Our  next  step  in  this  course  of  lectures  will  carry  us  into  the  micro- 
scopic world,  and  with  the  aid  of  the  lantern  at  the  next  lecture  I  shall 
hope  to  demonstrate  to  you  a  little  of  the  microscopic  structure  of  the 
body  and  of  the  general  nature  of  the  change,  which  exhibits  itself 
in  the  body  from  its  earliest  to  its  latest  condition.  With  such  knowl- 
ege  in  our  minds,  we  shall  be  able  next  to  study  some  of  the  laws  of 
growth.  We  shall  gain  from  our  microscopic  information  a  deeper 
insight  into  some  of  the  secrets  of  the  changes,  which  age  produces  in 
the  human  body. 


THE 

POPULAR    SCIENCE 

MONTHLY. 


AUGUST,  1907 


THE  PEOBLEM  OF  AGE,  GEOWTH  AND  DEATH 

By   CHARLES   SEDGWICK   MINOT,   LL.D.,   D.Sc. 

JAMES  STILLMAN  PROFESSOR  OF  COMPARATIVE   ANATOMY   IN   THE   HARVARD   MEDICAL  SCHOOL 

II.  Cytomorphosis.  The  Cellular  Ciiaxges  of  Age 
Ladies  and  Gentlemen:  1  endeavored  in  my  last  lecture  to  picture 
to  you,  so  far  as  words  could  suffice  to  make  a  picture,  something  of 
the  anatomical  condition  of  old  age  in  man,  and  to  indicate  to  you 
further  that  the  study  merely  of  those  anatomical  conditions  is  not 
enough  to  enable  us  to  understand  the  problem  we  are  tackling,  but 
that  we  must  in  addition  extend  the  scope  of  our  inquiry  so  that  it 
will  include  animals  and  plants,  for  since  in  all  of  these  living  beings 
the  change  from  youth  to  old  age  goes  on,  it  follows  that  we  can  hardly 
expect  an  adequate  scientific  solution  of  the  problem  of  old  age  unless 
we  base  it  on  broad  foundations.  By  such  breadth  we  shall  make  our 
conclusion  secure,  and  we  shall  know  that  our  explanation  is  not  of 
the  character  of  those  explanations  which  I  indicated  to  you  in  the  last 
lecture,  which  are  so-called  '  medical,'  and  are  applicable  only  to  man, 
but  rather  will  have  in  our  minds  the  character  of  a  safe,  soimd  and 
trustworthy  biological  conclusion.  The  problem  of  age  is  indeed  a 
biological  problem  in  its  broadest  sense,  and  we  can  not  study,  as  we 
now  know,  the  problem  of  age  without  including  in  it  also  the  con- 
sideration of  the  problems  of  growth  and  the  problems  of  death.  I 
hope  to  so  entice  you  along  in  the  consideration  of  the  facts,  which 
I  have  to  present,  as  to  lead  you  gently  but  perceptibly  to  the  con- 
clusion that  we  can  with  the  microscope  now  recognize  in  the  living 
parts  of  the  body  some  of  those  characteristics  which  result  in  old  age. 
Old  age  has  for  its  foundation  a  condition  which  we  can  actually  make 
visible  to  the  human  eye.  As  a  step  towards  tliis  conclusion,  I  desire 
to  show  you  this  evening  something  in  regard  to  the  microscopic  struc- 
ture of  the  human  body. 


98  POPULAR    SCIENCE   MONTHLY 

We  now  know  that  the  bodies  of  all  animals  and  plants  are  con- 
stituted of  minute  units  so  small  that  they  can  not  be  distinguished 
by  the  naked  eye,  although  they  can  be  readily  demonstrated  by  the 
microscope  These  units  have  long  been  known  to  naturalists  by  the 
name  of  cells      The  discovery  of  the  cellular  constitution  of  living 


Fig.  3.    Cells  from  the  Mouth  (Oral  Epithelium)  of  the  Salamander,  to  show  the 
phases  of  cell  division  or  mitosis. 

bodies  marks  one  of  the  great  epochs  in  science,  and  every  teacher  who 
has  had  occasion  to  deal  in  his  lectures  with  the  history  of  the  bio- 
logical sciences  finds  it  necessary  to  dwell  upon  this  great  discovery.  It 
was  first  shown  to  be  true  of  plants,  and  shortly  after  likewise  of 
animals.     The  date  of  the  latter  discovery  was  1839.     We  owe  it  to 


AGE,    GEO]yTH    AND    DEATH  99 

Theodor  Schwann,  whose  name  will  therefore  ever  be  honored  by  all 
investigators  of  vital  phenomena.  What  the  atom  is  to  the  chemist, 
the  cell  is  to  the  naturalist.  Every  cell  consists  of  two  essential  parts. 
There  is  an  inner  central  kernel  which  is  known  by  the  technical  name 
of  nucleus,  and  a  covering  mass  of  living  material  which  is  termed  the 
protoplasm  and  constitutes  the  body  of  the  cell.  I  will  now  call  for 
the  first  of  our  lantern  slides  to  be  thrown  upon  the  screen.  It  presents 
to  you  pictures  of  the  cells  as  they  are  found  lining  the  mouth  of 
the  European  salamander.  The  two  figures  at  the  top  illustrate  very 
clearly  the  elements  of  the  cell.  The  protoplasm  forms  a  mass,  ofPer- 
ing  in  this  view  no  very  distinctive  characteristics,  and  therefore  offer- 
ing a  somewhat  marked  contrast  with  the  nucleus  which  presents  in 
its  interior  a  number  of  granules  and  threads.  Every  nucleus  consists 
of  a  membrane  by  which  it  is  separated  from  the  protoplasm,  and  three 
internal  constituents :  First,  a  network  of  living  material,  more  or  less 
intermingled  with  which  is  a  second  special  substance,  chromatin, 
which  owes  its  name  to  the  very  marked  affinity  which  it  displays  for 
the  various  artificial  coloring  matters  which  are  employed  in  micro- 
scopical research.  The  third  of  the  internal  nuclear  constituents  we 
m.ay  call  the  sap,  the  fluid  material  which  fills  out  the  meshes  of  the 
network.  Later  on  we  shall  have  occasion  to  study  somewhat  more 
carefully  the  principal  variations  which  nuclei  of  different  kinds  may 
present  to  us,  and  we  shall  learn  from  such  study  that  we  may  derive 
some  further  insight  into  the  rapidity  of  development  and  the  nature 
of  the  changes  which  result  in  old  age.  While  the  picture  is  upon  the 
screen,  I  wish  to  call  your  attention  to  the  other  figures  which  illus- 
trate the  process  of  cell  multiplication.  As  you  regard  them  you  will 
notice  in  the  succession  of  illustrations  that  the  nucleus  has  greatly 
changed  its  appearance.  The  substance  of  the  nucleus  has  gathered 
into  separate  granules,  each  of  which  is  termed  a  chromosome.  These 
chromosomes  are  very  conspicuous  under  the  microscope,  because  they 
absorb  artificial  stains  of  many  sorts  with  great  avidity  and  stand  out 
therefore  conspicuously  colored  in  our  microscopic  preparations.  They 
are  much  more  conspicuous  than  is  the  substance  of  the  resting  nucleus. 
And  this  fact,  that  we  can  readily  distinguish  the  dividing  from  the 
resting  nucleus  under  the  microscope,  we  shall  take  advantage  of  later 
on,  for  it  offers  us  a  means  of  investigating  the  rate  of  growth  in 
various  parts  of  the  body.  I  should  like,  therefore,  to  emphasize  the 
fact  at  the  present  time  sufficiently  to  be  sure  that  it  will  remain  in 
your  minds  until  the  later  lecture  in  which  we  shall  make  practical 
use  of  our  acquaintance  with  it.  It  is  unnecessary  for  our  purposes 
to  enter  into  a  detailed  description  of  the  complicated  processes  of  cell 
division.  But  let  me  point  out  to  you  that  the  end  result  is  that 
where  we  have  one  cell  we  get  as  the  result  of  division — two;  but  the 


loo  POPULAR    SCIENCE   MONTHLY 

two  divided  cells  are  smaller  than  the  mother  cell  and  have  smaller 
nuclei.  They  will,  however,  presently  grow  up  and  attain  the  size  of 
their  parent. 

Every  cell  is  a  unit  hoth  anatomically  and  physiologically.  It  has 
a  certain  individuality  of  its  own.  In  many  cases  cells  are  found  to 
be  isolated  or  separated  completely  from  one  another.  But,  on  the 
other  hand,  we  also  find  numerous  instances  in  which  the  living  sub- 
stance of  one  cell  is  directly  continuous  with  that  of  another.  When 
the  cells  are  thus  related,  we  speak  of  the  union  of  cells  as  syncytium. 
Of  this  I  offer  you  an  illustration  in  the  second  picture  upon  the  screen, 
which  represents  the  embryonic  connective  tissue  of  man.  In  this  you 
can  see  the  prolongations  of  the  protoplasm  of  a  single  cell  body  uniting 
with  the  similar  prolongations  from  other  cell  bodies,  the  cells  them- 
selves thus  forming,  as  it  were,  a  continuous  network  with  broad  meshes 
between  the  connecting  threads  of  j)rotoplasm.  The  spaces  or  meshes 
are,  however,  not  entirely  vacant,  but  contain  fine  lines  which  corre- 
spond to  the  existence  of  fibrils,  which  are  characteristic  of  connective 
tissue  and  at  the  stage  of  development  represented  in  this  picture,  are 
beginning  to  appear.  It  is  fibrils  of  this  sort  which  we  find  as  the 
main  elements  in  the  constitution  of  sinews  and  tendons,  as,  for  in- 
stance, the  tendon  of  Achilles,  at  the  heel.  In  a  very  3^oung  body  we 
find  there  are  but  few  fibrils ;  in  the  adult  body  an  immense  number. 

There  is,  in  fact,  as  you  projDably  all  know,  a  constant  growth  of 
cells;  and  this  growth  implies  also,  naturally,  their  multiplication. 
There  has  been  in  each  of  us  an  immense  number  of  successive  cell 


Fig.  4.  Example  OF  A  Syncytium.  Embryonic  connective  tissue  from  the  urn  oilical  cord 
of  a  human  embryo  of  about  three  months,  magnified  about  400  diameters,  c,  c,  cells  ;  /,  inter- 
cellular fibrils. 


AGE,    GROWTH   AND    DEATH  loi 

generations,  and  at  the  present  time  a  multiplication  of  cells  is  going 
on  in  every  one  of  us.  It  never  entirely  ceases  as  long  as  life  continues. 
The  development  of  the  bod}^,  however,  does  not  consist  only  of  the 
growth  and  multiplication  of  cells,  but  also  involves  changes  in  the 
very  nature  of  the  cells,  alterations  in  their  structure.  Cells  in  us  are 
of  many  different  sorts,  but  in  early  stages  of  development  they  are  of 
few  sorts.  Moreover,  in  the  early  stages  we  find  the  cells  all  more  or 
less  alike.  They  do  not  differ  from  one  another.  Hence  comes  the 
technical  term  of  differentiation,  to  designate  the  modifications  which 
cells  undergo  with  advancing  age.  At  first  cells  are  alike;  in  older 
individuals  the  cells  have  become  of  different  sorts,  they  have  been 
differentiated  into  various  classes.      This  whole  phenomenon  of  cell 


■^  ri'i*j^-^~i«ii*-"=^-^-' 


i^ifi-r-ii...  :.-g.,,^^.^.:\:. 


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Fio.  5.  Three  Transverse  Sections  through  a  Rabbit  embryo  of  seven  and  one 
HALF  Days,  from  series  022  of  the  Harvard  Embryological  Collection.  A,  section  2J7  across  the 
anterior  part  of  the  germinal  area.  B,  section  200  across  the  middle  region  of  the  germinal 
area.    C,  section  381,  through  the  posterior  part  of  the  germinal  area.    Magnified  300  diameters 

change  is  comprehensively  designated  by  the  single  word,  cytomorphosis, 
which  is  derived  from  two  Greek  words  meaning  cell  and  form,  respect- 
ively. A  correct  understanding  of  the  conception  cytomorphosis  is  an 
indispensable  preliminary  to  any  comprehension  of  the  phenomena  of 


POPULAR   SCIENCE   MONTHLY 


development  of  animal  or  plant  structure.  I  shall  endeavor,  therefore, 
now  to  give  you  some  insight  into  the  phenomena  of  cytomorphosis  as 
regarded  by  the  scientific  biologist.  The  first  cells  which  are  produced 
are  those  which  form  the  young  embryo.  We  speak  of  them,  therefore, 
as  embryonic  cells,  or  cells  of  the  embryonic  type.  Our  next  picture 
illustrates  the  actual  character  of  such  cells  as  seen  with  the  microscope, 
for  it  represents  a  series  of  sections  through  the  body  of  a  rabbit 
embryo,  the  development  of  which  has  lasted  only  seven  and  one  half 
days.  You  will  notice  at  once  the  simplicity  of  the  structure.  There 
are  not  yet  present  any  of  those  parts  which  we  can  properly  designate 
as  organs.  The  cells  have  been  produced  by  their  own  multiplication 
and  are  not  yet  so  numerous  but  that  they  could  be  readily  actually 
counted.  They  are  spread  out  in  somewhat  definite  layers  or  sheets, 
but  beyond  that  they  show  no  definite  arrangement  which  is  likely  to 
attract  your  attention.  That  which  I  wish  you  particularly  to  observe 
is  that  in  every  part  of  each  of  these  sections  the  cells  appear  very  much 
alike.      The  nuclei  are  all  similar  in  character,  and  for  each  of  them 

there  is  more  or  less  protoplasm;  but  the 
protoplasm  in  all  parts  of  these  young 
rabbits  is  found  to  be  very  similar;  and 
indeed  if  we  should  pick  out  one  of  these 
cells  and  place  it  by  itself  under  the  micro- 
scope, it  would  be  impossible  to  tell  what 
part  of  the  rabbit  embryo  it  had  been 
taken  from,  so  much  do  all  the  cells  of 
all  the  parts  resemble  one  another.  We 
learn  from  this  picture  that  the  embryonic 
cells  are  all  very  much  alike,  simple  in 
character,  have  relatively  large  nuclei,  and 
only  a  moderate  amount  of  protoplasm 
for  each  nucleus  to  complete  the  cell. 

Very  difi'erent  is  the  condition  of 
affairs  which  we  find  when  we  turn  to 
the  microscopic  examination  of  the  adult. 
Did  time  permit  it  would  be  possible  to 
study  a  succession  of  stages  and  show  you 
that  the  condition  which  we  are  about  to 
study  as  existing  actually  in  the  adult  is 
the  result  of  a  gradual  progress  and  that 
in  successive  stages  of  the  individual  we 
can  find  successive  stages  of  cell  change; 
but  it  will  suffice  for  our  immediate  purpose  to  consider  the  results  of 
differentiation  as  they  are  shown  to  us  by  the  study  of  the  cells  of 


Fig.  6.  PoKTiON  OF  A  Trans- 
verse Section  of  the  Spinal  Cord 
OF  A  Human  Embryo  of  Four  Mil- 
limeters. Harvard  Embryological 
Collection,  series  714.  The  spinal 
cord  at  this  stage  is  a  tubular  struc- 
ture. The  figure  shows  a  portion  of 
the  wall  of  the  tube ;  the  lefthand 
boundary  of  the  figure  corresponds 
to  the  inner  surface  of  the  tube. 


AGE,    GROWTH   AND    DEATH 


103 


the  adult.  I  will  have  thrown  upon  the  screen  for  you  a  succession 
of  pictures  illustrating  various  adult  structures.  The  first  is,  how- 
ever, a  section  of  the  embryonic  spinal  cord  in  Avhich  you  can  see 
that  much  of  the  simple  character  of  the  embryonic  cells  is  still  kept. 
All  parts  of  the  spinal  cord,  as  the  picture  shows,  are  very  much 
alike,  and  the  nuclei  of  the  cells  composing  the  spinal  cord  at  this 
stage  are  all  essentially  similar  in  appearance.  What  a  contrast  this 
forms  with  our  next  picture,  which  shows  us  an  isolated  so-called 
motor  nerve  cell  from  the  adult  spinal  cord.  It  owes  its  name  motor 
to  the  fact  that  it  produces  a  nerve  fiber  by  which  motor  impulses 

are  conveyed  from  the 
spinal  cord  to  the  mus- 
cles of  the  body.  The 
j^  cell  has  numerous  elon- 
gated branching  proc- 
esses stretching  out  in 
various  directions,  but  all 
leading  back  towards  the  cen- 
tral body  in  which  the  nucleus 
is  situated.  These  are  the 
processes  which  serve  to  carry 
in  the  nervous  impulses  from 
the  periphery  towards  the 
center  of  the  cell,  impulses 
which  in  large  part,  if  not  ex- 
clusively, are  gathered  up  from 
other  nerve  cells  which  act  on 
the  motor  element.  At  one 
point  there  runs  out  a  single 
process  of  a  different  char- 
acter. It  is  the  true  nerve 
filjer,  and  forms  the  axis,  as  it 
was  formerly  termed ;  or  axon, 
as  it  is  at  present  more  usually 


Fig.  7.    Copy  of  the  Okiginal  Figure  from 
THE  Memoir  of  deiters,  in  which  the  proof  of    named,  of  the  nerve  fiber   as 

the  origin  of  the  nerve  fibers  directly  irom  the 
nerve  ceUs  was  first  published.  The  raemoir  is  one 
of  the  classics  of  anatomy.  It  was  issued  posthu- 
mously, for  the  author  died  young  to  the  great  loss 
of  science.  The  figure  represents  a  single  isolated 
motor  nerve  cell  from  the  spinal  cord  of  an  ox. 
The -ingle  unbrunched  axon  Ax,  is  readily  distin- 
guished from  the  multiple  branching  dendrites. 


we  encounter  it  in  an  ordinary 
nerve.  This  single  thread- 
like prolongation  of  the  nerve 
cell  is  likewise  constituted  by 
the     living     protoplasm     and 


serves  to  carry  the  impulses 
away  from  the  cell  body  and  transmit  them  ultimately  to  the  muscle 
fibers  which  are  to  be  stimulated  to  contraction.     In  the  embrj'onic 


I04 


POPULAR   SCIENCE   MONTHLY 


Fig.  8  A  Large  Cell  from  the  Small  Brain  (Cerebellum)  of  a  Man.  It  is  usually 
called  a  Puikinje's  cell.  It  was  stained  black  throughout  by  what  is  known  as  the  Golgi  silver 
method,  hence  shows  nothing  of  its  internal  structure.    After  von  Kollikt-r. 

spinal  cord  none  of  these  processes  existed,  and  the  amount  of  the 
protoplasm  in  the  nerve  cell  was  very  much  smaller.  As  develop- 
ment progressed,  not  only  did  the  protoplasm  hody  grow,  but  the 
processes  gradually  grew  out.  Some  of  them  branched  so  as  to  better 
receive  and  collect  the  impulses;  one  of  them  remained  single  and 
very  much  elongated,  and  acquired  a  somewhat  different  structure  in 
order  to  serve  to  carry  the  nervous  impulses  away.  The  third  picture^ 
shows  us  a  section  through  the  spinal  cord  of  an  adult  fish.  It  has 
been  treated  by  a  special  stain  in  order  to  show  how  certain  elements 
of  the  spinal  cord  acquire  a  modification  of  their  organization  by  which 
•  they  are  adapted  to  serve  as  supports  for  the  nervous  elements  proper. 
They  play  in  the  microscopic  structure  the  same  supporting  role  which 
the  skeleton  performs  in  the  gross  anatomy  of  the  body  as  a  whole. 
They  do  not  take  an  active  part  in  the  nervous  functions  proper. 
None  of  the  appearances  which  this  figure  offers  for  our  consideration 
can  be  recognized  in  any  similar  preparation  of  the  embryonic  cord. 
Obviously,  then,  from  the  embryonic  to  the  adult  state  in  the  spinal 
cord  there  occurs  a  great  differentiation.  That  which  was  alike 
in  all  its  parts  has  been  so  changed  that  we  can  readily  see  that 
it  consists  of  many  different  parts.  A  striking  illustration  of  this 
is  afforded  by  the  next  picture,  which  represents  one  of  the  large 
nerve  cells  which  occur  in  the  small  brain,  or  cerebellum,  that  portion 
of  the  central  nervous  system  which  the  physiologists  have  demon- 


^  The  illustration  referred  to  is  not  reproduced  in  the  text. 


AGE,    GI10\\TH    AND    DEATH 


105 


strated  to  be  particularly  concerned  in  the  regulation  and  coordination 
of  movements.      These  large  cells  occur  only  in  this  portion  of  the 


Jus,      yf. 


fVf/  y. 


jfj?*-     ■.%1i 


■\> 


& 


"^ 


^ 


% 


9    ^ 


ipr'^il^ 


\ 


hi/ 


(^ 


Fifl/L 


l-'i'l  'i 


Fif.}.  J 


Fii;.  y     Vakiois  Kinds  of  Human  Nkkvk  V,y.\.\.- 
Arier  Sob"lla. 


DKSCKII'.KIi    IN    TliK   TKXT. 


brain,  and,  as  you  see,  differ  greatly  in  appearance  from  the  motor  cells 
of  the  type  which  we  were  considering  a  few  moments  ago.     And,  again, 


io6 


POPULAR   SCIENCE   MONTHLY 


another  picture  illustrates  yet  other  peculiarities  of  the  adult  nerve 
cells.  The  upper  figures  in  this  plate  are  taken  from  cells  which  have 
been  colored  uniformly  of  a  very  dark  hue,  in  consequence  of  which 


(T. 


(^> 


0 


©•I'^*®  'f»>  -,.   "'**^-  ^  ^'  /Sits 


I'lff.l. 


^ 


Fiff.£. 


^*>&& 


Ff  (/..')'. 


Fi\g.  4 


Pig.  10.    Sections  of  Four  Sokts  of  Epithelium.     Alier  Swbntta. 

they  are  rendered  so  opaque  that  the  nucleus  which  they  really  contain 
is  hidden  from  our  view.  But  the  deep  artificial  color  makes  it  easy 
to  follow  out  the  form  of  the  cells  and  the  ramifications  of  their  long 
processes.  In  the  middle  figures  we  have  cells  which  have  been  stained 
by  another  method  which  brings  out  very  clearly  to  the  eye  the  fact 


AGE,    GROMTH    AND    DEATH 


107 


that  in  the  protoplasm  of  the  cell  there  are  scattered  spots  of  substance 
of  a  special  sort,  is^o  such  spots  can  be  demonstrated  in  the  elements 
of  the  young  embryonic  nerve  cells.  To  some  fanciful  observers  the 
spots,  thus  microscopically  demonstrable  in  the  nerve  cells,  recall  the 
spots  which  appear  on  the  skin  of  leopards,  and  hence  they  have  be- 
stowed upon  these  minute  particles  the  term  tigroid  substance.  The 
bottom  figures  represent  the  kind  of  nerve  cells  which  occur  upon  the 
roots  of  the  spinal  nerves.  It  is  unnecessary  to  dwell  upon  their  ap- 
pearance, as  the  mere  inspection  of  the  figures  shows  at  once  that  they 
differ  very  much  indeed  from  the  other  nerve  cells  we  have  considered. 
We  pass  now  to  another  group  of  structures,  the  tissues  which  are 
known  by  the  technical  name  of  epithelia.  You  can  notice  immediately 
in  the  figures  from  the  skin  that  the  appearances  are  very  different 
from  those  we  have  encountered  in  contemplating  the  cells  of  the 
nervous  system.  And  you  can  readily  satisfy  yourselves  by  the  com- 
parison with  the  various  figures  now  before  you,  of  the  fact  that  these 
epithelia  are  unlike  one  another.  The  figures  represent  epithelium, 
respectivel}^,  first  from  the  human  ureter;  second,  from  the  respiratory 
division  of  the  human  nose;  third,  from  the  human  ductus  epidid3miidis, 
and  fourth,  from  the  pigment  layer  of  the  retina  of  the  cat.  We  turn 
now  to  a  representation  of  a  section  of  one  of  the  orbital  glands. 
This  is  very  instructive  because  we  see  not  only  that  the  cells  which 
compose  the  gland  have  acquired  a  special  character  of  their  own,  but 
also  that  they  are  not  uniform  in  their  appearances.  This  lack  of 
uniformity  is  due  chiefly  to  the  fact  that  the  cells  change  their  appear- 
ance according  to  their  functional  state.  We  can  actually  see  in  these 
cells  under  the  microscope  the  material  imbedded  in  their  protoplasmic 


A  Ji 

Fig.  11.  To  snow  tiik  Okbitai,  Glands,  A,  with  the  inrtterinl  to  form  the  secretion 
acfMJraulateU  within  the  cells.  /?,  after  loss  of  the  material  through  prolonf^ed  secretion. 
P'rorn  R.  Hei^lenbain  after  Lavdowsky. 


io8 


POPULAR    SCIENCE   MONTHLY 


bodies  out  of  which  the  secretion,  which  is  to  be  poured  forth  by  the 
cells,  is  to  be  manufactured.  So  long  as  that  material  for  the  secretion 
is  contained  in  the  cells,  the  cells  appear  large,  and  their  protoplasmic 
bodies  do  not  readily  absorb  certain  of  the  staining  matters,  which  the 
microscopist  is  likely  to  apply  to  them.  When,  however,  the  accumu- 
lated raw  material  has  been  changed  into  the  secretion  and  discharged 
from  the  gland,  the  cell  is  correspondingly  reduced  in  bulk,  and  as  you 
see  in  this  figure,  it  then  takes  up  the  stain  with  considerable  avidity, 
as  does  also  the  nucleus  which  has  likewise  become  reduced  in  size. 
These  facts  are  very  instructive  for  us,  since  they  prove  conclusively 
that  with  the  microscope  we  can  see  at  least  part  of  the  peculiarities  in 
cells  which  are  correlated  with  their  functions.  We  can  actually  ob- 
serve that  the  cells  of  the  salivary  glands  are  able  to  produce  their 
peculiar  secretion  because  they  contain  a  kind  of  substance  which  in 
the  embryonic  cell  does  not  appear  at  all.  There  is  a  visible  differen- 
tiation of  these  salivary  cells  from  the  simple  stage  of  the  embryonic 
cells.  Something  similar  to  this  can  be  recognized  in  the  next  of  our 
pictures  representing  a  section  of  the  gland  properly  known  as  the 
pancreas,  but  which  is  sometimes  termed  the  abdominal  salivary  gland 
for  the  reason  that  it  somewhat  resembles  the  true  salivary.  In  the 
cells  of  the  pancreas  also  we  can  see  the  material,  which  is  to  produce 
the  secretion,  accumulated  in  the  inner  portion  of  the  cell,  and  when  it 
is  so  accumulated  the  cell  appears  enlarged  in  size  and  the  nucleus  is 
driven  back  towards  the  outer  end  of  the  cell  where  some  unaltered 
protoplasm  is  also  accumulated.      When  this  raw  material  is  turned 


Pig.  12.  Two  Sections  of  the  Pancreatic  Gland  of  a  Dog.  A,  the  cells  are  enlarged 
by  the  accumulation  of  material  to  form  the  secretion.  B,  the  cells  are  shrunk  because  there 
has  been  prolonged  secretion  and  part  of  their  substance  is  lost.    From  R.  Heidenhain. 


AGE,    GRO}YTH    AND    DEATH 


109 


over  into  secretion  by  a  chemical  change,  it  is  discharged  from  the  cell, 
the  cell  loses  in  volume  and  in  its  shrunken  state  presents  a  very  dif- 
ferent appearance,  as  is  shown  at  B  in  the  figure.  It  is  necessary  for 
the  cells  to  again  elaborate  the  material  for  secretion  before  they  can  a 
second  time  become  functionally  active.  Here  we  have  somethmg  of 
the  secret  of  the  production  of  the  various  juices  in  the  body  revealed 
to  us.  Other  excellent  examples  of  the  differentiated  condition  of  the 
cells  are  afforded  us  bv  the  examination  of  hairs,  of  which  I  will  show 
you  two  i^ictures.      The  first  represents  a  section  through  the  human 


Fig.  13.    SiXTioN  OF  Tin-,  IUman  Skin,  mai,k  so  that  the  Hairs  akf.  cut  Lengthwise. 

skin  taken  in  such  a  way  that  the  hairs  are  themselves  cut  lengthwise 
and  you  can  see  not  only  that  each  hair  consists  of  various  parts,  but 
also  that  the  cells  in  these  parts  are  unlike.  The  follicles  within  the 
skin  in  which  the  hair  is  lodged  likewise  have  walls  with  cells  of  various 
6orts.  It  may  interest  you  also  to  point  out  in  the  figure  the  little 
muscle  which  runs  from  each  hair  to  the  overlying  skin,  so  disposed 
that  when  the  muscle  contracts  the  "  particular  hair  will  stand  up  on 


no 


POPULAR    SCIENCE   MONTHLY 


end."  Still  more  clearly  does  the  variety  of  cells  which  actually  exists 
in  a  hair  show  in  the  following  picture,  which  represents  a  cross-section 
of  a  hair,  and  its  follicle,  hut  more  highly  magnified  than  were  the 
hairs  in  the  previous  figure.  The  adult  body  consists  of  numerous 
organs.      These  are  joined  together  and  kept  in  place  by  intervening 


''//^/    '\,^ 


w 


^•■5l.:  ■■','••»'/'*' 


/^f/ 


Fig.  14.    Ckoss  Section  of  the  Root  of  a  Hair. 

substance.  The  organs  themselves  consist  of  many  separate  parts  which 
are  also  joined  by  a  substance  which  keeps  them  in  place.  This  sub- 
stance has  received  the  appropriate  name  of  connective  tissue.  We 
find  in  the  adult  that  it  consists  of  a  considerable  number  of  structures- 
There  are  cells  and  fibers  of  more  than  one  kind,  which  have  been  pro- 
duced by  the  cells  themselves.  There  is  more  or  less  substance  secreted 
by  the  cell  which  helps  to  give  consistency  to  the  tissue.  In  some  cases 
this  substance  which  is  secreted  by  the  cells  becomes  tougher  and  ac- 
quires a  new  chemical  character.  Such  is  the  case,  for  instance,  with 
cartilage.  Or,  again,  you  may  see  a  still  greater  chemical  meta- 
morphosis going  on  in  the  material  secreted  by  the  cells  in  the  case  of 
bone,  where  the  substance  is  made  tougher  and  stronger  by  the  deposit 


AGE,    GROWTH   AXD    DEATH 


of  calcareous  material.  Nothing  like  cartilage,  nothing  like  bone,  exists 
in  the  early  state  of  the  embryo.  They  represent  something  different 
and  new.  The  next  of  our  illustrations  shows  us  a  muscle  fiber  of  the 
sort  which  serves  for  our  voluntary  motions,  which  is  connected  typ- 
ically with  some  part  of  the  skeleton.  These  muscle  fibers  are  elon- 
gated structures.  Each  fiber  contains  a  con- 
tractile substance  different  from  protoplasm,  and  |  •■  -  ~- 
which  exists  in  the  form  of  delicate  fibrils  which 
run  lengthwise  in  the  muscle  fibers,  and  is  so 
disposed,  further,  that  a  series  of  fine  lines  are 
produced  across  the  fiber  itself,  each  line  cor- 
responding with  a  special  sort  of  material  dif- 
ferent from  the  original  protoplasm.  These 
cross  lines  give  to  the  voluntary  muscle  fibers 
a  very  characteristic  appearance,  in  consequence 
of  which  they  are  commonly  designated  in 
scientific  treatises  by  the  term  striated.  A 
striated  muscle  fiber  is  that  which  is  under  the 
control  of  our  will.  It  should  perhaps  be  men- 
tioned that  the  muscle  fibers  of  the  heart  are 
also  striated,  though  they  differ  very  much  in 
other  respects  from  the  true  voluntary  muscles. 
And  last  of  all  for  this  series  of  demonstrations, 
I  have  chosen  a  representation  of  the  retina. 
One  can  see  at  the  top  of  the  figure  the  peculiar 
cylindrical  and  developing  projections,  which 
are  characteristic  of  a  retina,  projections  which 
are  of  especial  interest  because  they  represent 
the  apparatus  by  which  the  rays  of  light  are 
transformed  into  an  actual  sensory  perception. 

After  this  has  been  accomplished,  the  perception  is  transmitted  into 
the  interior  substance  of  the  retina,  and  by  the  complication  of  the 
figure  you  may  judge  a  little  of  the  complication  of  the  arrangements 
by  which  the  transmission  through  this  sensory  organ  is  achieved,  until 
the  perception  is  given  off  to  a  nerve  fiber  and  carried  to  the  brain. 
There  is  not  time  to  analyze  all  I  might  present  to  you  of  our  present 
knowledge  concerning  the  structure  of  the  retina.  But  it  will,  I  think, 
suffice  for  purposes  of  illustration  to  call  your  attention  to  the  com- 
plicated appearance  of  the  section  as  a  whole  and  to  assure  you  that 
nothing  of  the  sort  exists  in  the  early  stage  of  the  embryo.  To  re- 
capitulate, then,  what  we  have  learned  from  the  consideration  of  these 
pictures,  we  may  say  that  in  place  of  uniformity  we  now  have  diversity. 
It  should  be  added,  to  make  the  story  complete,  that  the  establishment 
of  this  diversity  has  been  gradually  brought  about,  and  that  that  which 


Fig.  15.  Part  of  a 
MU.SCLE  Fiber  of  the 
Human  ToNcrE  to  show 
THE  Cross  Striations. 
Two  nuclei  are  included, 
one  of  which  is  shown  at 
the  edge  of  the  fiber,  the 
other  in  surface  view. 
In  the  adult  striated 
muscle  fibers  of  mam- 
mals the  nuclei  are  su- 
perficially placed. 


POPULAR   SCIENCE   MONTHLY 


Blood  vessels. 


ment. 

ili-—  Cone,  outer  seg- 

A^  ment. 

IP;""*  Cone,  inner  seg- 

'^^l  ruent. 

is^^'^v^  Rod,    inner  seg- 

O  ment. 

^' 

fg^yji."'^!^]  Base    of  a   cone 


Nucleus. 


Nucleus. 


Inner  surface  ot 
the  retina  (to- 
ward ttie  light). 


Fibers  which 
pass  inio  the 
optic  nerve. 

Blood  vessels 
Fig.  16.  Section  of  a  Human  Retina,  from  Stohr's  Histology,  sixth  American  edition. 
Aithough  the  retina  is  very  thin  it  comprises  no  le?s  than  twelve  distinct  layers ;  the  outermost 
layer  is  highly  vascular.  The  pigment  layer  prevents  the  escape  of  light.  The  rods  and  cones 
convert  the  light  waves  into  a  sensory  impulse,  which  is  transmitted  through  the  remaining 
layers  of  the  retina  to  the  optic  nerve.    The  total  structure  is  extremely  complicated. 

we  call  develoiDiiient  is  in  reality  nothing  more  than  the  making  of 
diversity  out  of  uniformity.  It  is  a  process  of  differentiation.  Dif- 
ferentiation is  indeed  the  fundamental  phenomenon  of  life;  it  is  the 
central  problem  of  all  biological  research,  and  if  we  understood  fully 
the  nature  of  differentiation  and  the  cause  of  it,  we  should  have 
probably  got  far  along  towards  the  solution  of  the  final  problem  of 
the  nature  of  life  itself. 

The  size  of  animals  deserves  a  few  moments  of  our  time,  for  it  is 
intimately  connected  with  our  problem  of  growth  and  differentiation. 
Cells  do  not  differ  greatly  from  one  another  in  size.  The  range  of 
their  dimensions  is  very  limited.  This  is  particularly  true  of  the  cells 
of  any  given  individual  animal.  Eecent  careful  investigations  have 
been  made  upon  the  relation  of  the  size  of  cells  to  the  size  of  animals, 
and  it  has  been  found  that  animals  are  not  larger,  one  than  another, 
because  their  cells  are  larger,  but  because  they  have  more  of  them. 
This  statement  must  be  understood  with  certain  necessary  reservations. 
There  are  some  kinds  of  animals,  like  the  star-fish,  which  have  very 
small  cells;  others,  like  frogs  and  toads,  which  have  large  cells;  so 
that  a  star-fish  of  the  same  bulk  as  a  given  frog  would  contain  a  great 
many  more  cells.  Our  statement  is  true  of  allied  animals.  For  ex- 
ample, a  large  frog  differs  from  a  small  'frog,  or  a  large  dog  from  a 
small  dog  by  the  number  of  the  cells.  An  important  exception  to  this 
law  is  offered  for  our  consideration  by  the  cells  of  the  central  nervous 


AGE,    GEO}yTH    AND    DEATH 


113 


system,  the  nerve  cells  properly  so  called.  This  is  demonstrated  by  the 
slide  now  before  ns,  which  shows  us  corresponding  motor  nerve  cells 
of  twelve  different  animals  arranged  in  the  order  of  their  size — the 
elephant,  the  cow,  the  horse,  man,  the  pig,  the  dog,  the  baboon,  the  cat, 
the  rabbit,  the  rat.  the  mouse,  and  a  small  bat.     You  recognize  im- 


El«^plum  iiuli(Mi>s 
84.1x71.5 


lio.s  laurii.H     Evim.«<  cnlHillii.*? 
7L'.4xr,G.7  07.»>'."(;.7 


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Homo 
07.r>x.^4.O 


03.4x51.  :j 


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(yiiocej»h(ilii.'<  iMihiiiii    Felix  domestical    Lepu.'^niniciilii.^  doiiH'slicii.s 
G().7xr>G.3  .5«.()x54.o  4'kr.x;i«;.4 


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Mils  rnllus  ulbus       Miis  inii.srnlu^ciibu.'^  AlSlapJio  dnfrea 
37.Hx:^:v.7  :i<-..ftxL»2.j)  31.5x28.0 


FxG.  17.    Motor  Nerve  Cells  of  Various  Mammals,  all  from  the  cervical  region  of  the 
spinal  cord.    The  cells  are  represented,  all  unilormly  magnified.    After  Irving  Htudesty. 

mediately  that  there  is  a  proportion  l)etweon  the  size  of  these  cells 
and  the  size  of  the  respective  species  of  animals.  To  a  minor  degree, 
but  much  less  markedh^,  there  is  a  difference  in  the  caliber  and  length 
of  the  muscle  fibers.  But  with  tliese  exceptions  our  statement  is  very 
nearly  exactly  true,  that  llic  diirci-cnce  in  size  of  animals  docs  not  in- 
volve a  difference  in  ilie  size  of  tlicii'  cells.  For  the  ])ur|)osc  of  the 
study  of  devclopincnj,  which  we  arc  to  make  in  these  lectures,  this  uni- 


114 


POPULAR   SCIENCE   MONTHLY 


fcrmity  in  the  size  of  cells  is  a  great  advantage,  and  enables  us  to  speak 
in  general  terms  in  regard  to  the  growth  of  cells,  and  renders  it 
superfluous  to  stop  and  discuss  for  each  part  of  the  body  the  size  of 
the  cells  which  compose  it,  or  to  seek  to  establish  different  principles 
for  different  animals  because  their  cells  are  not  alike  in  size. 

ISTow  we  pass  to  a  totally  different  aspect  of  cell  development,  that 
which  is  concerned  with  the  degeneration  of  cells.     For  we  find  that, 


../H,;^^^  t 


.. ...  ,j 


^^"  '"v,  '■::} 


-1. 


B 


Fig.  18.    Changes  in  the  Nerve  Cells  with  Age. 


after  the  differentiation  has  been  accomplished,  there  is  a  tendency  to 
carry  the  change  yet  further  and  to  make  it  so  great  that  it  goes 
beyond  perfection  of  structure,  so  far  that  the  deterioration  of  the 
cell  comes  as  a  consequence.  Such  cases  of  differentiation  we  speak  of 
as  a  degeneration,  and  it  may  occur  in  a  very  great  number  of  ways. 
Very  frequently  it  conies  about  that  the  alteration  in  the  structure  of 
the  cell  goes  so  far  in  adapting  it  to  a  special  function  that  it  is  unable 
to  maintain  itself  in  good  physiological  condition,  and  failing  to  keep 
up  its  own  nourishment  it  undergoes  a  gradual  shrinkage  which  we 
call  atrophy.  A  very  good  illustration  of  this,  and  a  most  important 
one,  is  offered  us  by  the  changes  which  go  on  in  the  nerve  cells  in 
extreme  old  age.  This  is  beautifully  illustrated  by  the  two  pictures 
which  are  now  before  us,  copied  from  investigations  of  Professor  Hodge, 
of  Clark  University.  The  two  figures  represent  human  nerve  cells 
taken  from  the  root  of  a  spinal  nerve.  The  left-hand  figure  shows 
these  cells  as  they  exist  in  their  full  maturity;  the  right-hand 
figure,  as  they  appear  in  a  person  of  extreme  old  age.  In  the  latter 
you  will  readily  notice  that  the  cells  have  shrunk  and  no  longer  fill 


AGE,    GROWTH   AND   DEATH  115 

the  spaces  allotted  to  them,  the  nuclei  have  hecome  small,  and  the 
protoplasm  has  changed  its  appearance  very  strikingly  because  there 
have  been  deposited  in  it  granules  of  the  pigment  which  impart  to  these 
cells  an  appearance  very  different  from  that  which  they  had  in  their 
maturity  when  their  functional  powers  were  at  their  maximum.  You 
will  notice  also  in  other  parts  of  the  right-hand  figure  that  the  atrophy 
of  the  cells  has  led  on  to  their  disintegration,  that  they  are  breaking 
down,  being  destroyed,  and  that  the  result  of  their  breaking  down  will 
ultimately  be  their  disappearance.  Thus  the  atrophy  of  a  cell  may 
lead  to  its  death.  The  other  two  figures^  upon  the  screen  show  us  the 
brain  of  the  humljle  bee.  On  the  left  is  the  brain  of  the  bee  in  the 
condition  in  which  we  find  it  when  the  bee  first  emerges  from  the 
pupa  or  chrysalis.  The  cells  are  then  in  a  fine  physiological  condition, 
but  in  a  few  weeks  at  most  the  bee  becomes  old  and  in  the  space  which 
belongs  to  each  cell  we  find  only  its  shrunken  and  atrophied  remnants, ' 
the  nucleus  greatly  reduced  in  volume,  and  an  irregular  mass  of  proto- 
plasm shrunk  together  around  it.  These  cells  have  likewise  under- 
gone an  atrophy  and  are  on  their  way  to  death.  In  other  cases  we 
find  that  there  is  a  change  going  on  which  we  call  necrobiosis,  which 
means  that  the  cells  continue  to  live,  but  change  their  chemical  organi- 
zation so  that  their  substance  passes  from  a  living  to  a  dead  state.  'Ro 
more  perfect  illustration  of  this  sort  of  change  can  be  found  than  that 
which  is  afforded  by  the  skin.  In  the  deep  layer  of  the  outer  skin  are 
the  living  and  growing  parts,  which  we  all  know  from  experience  are 
sensitive.  As  these  multiply  some  of  them  move  up  towards  the  sur- 
face ;  and  they  are  continually  shoved  nearer  and  nearer  the  surface  by 
the  growth  of  the  cells  underneath.  They  finally  become  exposed  at 
the  surface  by  the  loss  of  the  superficial  cells  which  preceded  them. 
During  this  migration  the  protoplasm  of  each  cell,  which  was  alive,  is 
changed  chemically  into  a  new  substance  which  we  call  keratin,  or  in 
common  language,  horny  substance.  Ultimately  the  cell  protoplasm 
becomes  nothing  but  horny  substance  and  is  absolutely  dead.  Here 
life  and  death  play  together  and  go  hand  in  hand.  Hence  the  term 
necrobiosis,  life  and  death  in  one.  Another  form  of  degeneration  which 
occurs  in  many  cases  is  of  great  interest  because  it  seems  as  if  the  cells 
were  making  a  last  great  effort;  and  their  final  performance  is  one 
of  enlargement.  They  become  greater  in  size  than  before;  but  there 
will  follow  a  disintegration  of  these  cells  also;  and  they  break  down 
and  are  lost.  This  form  of  degeneration  is  termed  hypertrophic,  and 
represents  a  third  type,  as  I  have  stated.  In  all  parts  of  the  body 
degenerative  changes  are  going  on,  and  they  represent  collectively  a 
third  phase  in  the  cytomorphic  cycle.  But  there  is  yet  one  more  phase, 
which  is  needed  to  complete  the  story.     That   is  the  phase  of  the 

The  two  figures  of  the  bee's  brain  are  not  reproduced  in  the  text. 


ii6  POPULAR   SCIENCE   MONTHLY 

death  and  final  removal  of  the  cells.     The  degenerative  change  always 
results  in  the  death  of  the  cell.     In  many  cases  the  dead  material  is 
removed  merely  by  being  cast  off,  as  is  the  case  with  the  skin.     All  the 
scales  which  peal  off  from  the  outer  surface  of  our  body  represent  little 
scraps  or  clusters  of  cells  which  are  entirely  dead ;  and  in  the  interior 
of  the  body,  in  the  intestinal  canal,  and  in  the  glands  of  the  stomach, 
we  find  cells  continually  dying,  dropping  off  from  their  place  upon  the 
walls,  and  being  cast  away.     Or  if  we  examine  the  saliva  which  comes 
from  the  mouth,  we  detect  that  that  also  is  full  of  cells  which  have 
died  and  fallen  off  from  their  connection  with  the  body  and  are  thus 
removed.     An  even  more  important  method  of  the  removal  of  cells  is 
by  a  chemical  process  in  consequence  of  which  the  cells  are  dissolved 
and  disappear  before  our  eyes,  very  much  as  marble  may  disappear 
from  sight  under  the  corrosive  action  of  an  acid.     Indeed,  we  know 
that  all  the  parts  of  the  body,  so  far  as  they  are  alive,  produce  within 
themselves  a  ferment  which  has  a  tendency  to  destroy  the  living  sub- 
stance itself.     The  production  of  these  destructive  agents  is  going  on  at 
all  times,  apparently,  in  all  parts  of  the  body,  which  are  alive.     A 
striking  illustration  of  this  is  offered  in  the  stomach.     The  digestive 
juice  which  is  produced  in  the  stomach  is  capable  of  attacking  and 
destroying  living  substance,  and  any  organic  material  suitable  for  food 
which  is  placed  in  the  stomach  will,  as  we  know,  be  attacked  by  the 
gastric  juices,  dissolved  to  a  certain  extent  by  them,  and  so  destroyed. 
Why  then  does  the  gastric  juice  not  attack  the  stomach  itself?     This  is 
but  one   phase   of   the   problem   why   the   body   does   not   continually 
destroy  itself.     It  has  lately  been  ascertained  by  some  ingenious  phys- 
iological investigations  that  the  body  not  only  produces  the  destructive 
agents,   but  also   antagonists   thereto,   anti-compounds   which   tend   to 
prevent  the  activity  of  the  destroying  factors.     The  whole  problem  is 
one  of  great  interest  and  importance  which  calls  for  very  much  further 
investigation  before  we  can  be  said  to  have  arrived  at  a  clear  under- 
standing of  it.     But  it  helps  us  much  in  our  conception  of  eytomor- 
phosis  to  know  that  all  portions  of  the  body  are  endowed  with  this 
faculty  of  destroying  themselves,  for  it  enables  us  to  understand  how  it 
is  possible  that  after  the  degeneration  of  a  cell  it  will  be  dissolved  away. 
It  is  merely  that  the  agents  of  solution  which  are  ordinarily  held  at 
bay  are  no  longer  restrained,  and  they  at  once  do  their  Avork.     There  is 
another,  but  comparatively  rare,  mode  of  cell-destruction.      The  cells 
break  up  into  separate  fragments,  which  are  then  dissolved  by  chemical 
means  and  disappear,  by  the  method  of  histolysis  above  described,  or 
else  are  devoured  by  the  cells,  to  which  reference  was  made  in  the 
first  lecture,  and  which  are  known  by  the  name  of  phagocytes,  and  to 
which  Metchnikoff  has  attributed  so  great  an  importance.     It  is  un- 
questionable that  phagocytes  do  eat  up  fragments  of  cells  and  of  tissues, 


AGE,    GROWTH   AND    DEATH 


117 


and  may  even  attack  whole  cells.  But  to  me  it  seems  probable  that 
their  role  is  entirely  secondary.  They  do  not  cause  the  death  of  cells, 
but  they  feed  presumably  only  upon  cells  which  are  already  dead  or  at 
least  dying.  Their  activity  is  to  be  regarded,  so  far  as  the  problem 
of  the  death  of  cells  is  concerned,  not  as  indicating  the  cause  of  death, 
but  as  a  phenomenon  for  the  display  of  which  the  death  of  the  cell  offers 
an  opportunity.  The  subject  of  the  death  and  disintegration  of  cells 
i.-  an  exceedingly  complex  one,  and  might  well  occupy  our  attention  for 
a  long  time.  But  it  is  not  permissible  to  depart  from  the  strict  theme 
which  we  have  before  us,  and  I  will  content  myself,  therefore,  with 
throwing  upon  the  screen  two  tables^  which  illustrate  to  us  the  varia- 
tions in  the  death  of  cells  and  in  their  modes  of  removal  which  are 


First. 
A. 


c. 

Second, 
A. 


B. 


Third. 
A. 
B. 

C. 


- 1.     Death  of  Cells 
Causes  of  death. 
External  to  the  organism : 

(1)  Physical    (mechanical,  chemical,  thermal,  etc.). 

(2)  Parasites. 

Changes  in  intercellular  substances   (probably  primarily  due  to  cells)  : 

( 1 )  Hypertrophy. 

(2)  Induration. 

(3)  Calcification. 

(4)  Amyloid  degeneration    (infiltration). 
Changes  inherent  in  cells : 

Morphological  changes  of  dying  cells. 
Direct  death  of  cells : 

(1)  Atrophy. 

(2)  Disintegration  and  resorption. 
Indirect  death  of  cells: 

(1)  Necrobiosis   (structural  change  precedes  final  death). 

(2)  Hypertrophic   aegeneration    (growth   and   structural   change   often 

with  nuclear  proliferation  precede  final  death). 
Removal  of  cells. 

By  mechanical  means    (sloughing  or  shedding) 
By  chemical  means   (solution). 
By  phagocytes. 


II.     Indirect  Death  of  Cells 
Necrobiosis. 

( 1 )  Cytoplasmic  changes : 
(a)    Granulation. 

(h)    Hyaline  transformation. 

(c)  Imbibition. 

(d)  Desiccation. 

(e)  Blasmatosis. 

(2)  Nuclear  changes: 
(a)  Karyorhe.\is. 
(h)    Karyolysis. 

Hypertrophic  degeneration. 

(1)  Cytoplasmic: 

(a)  Granular. 

(b)  Cornifying. 

(c)  Hyaline. 

(2)  Paraplasmic: 
(a)    Fatty. 

(5)    Pigmentary. 
(c)    Mucoid. 
id)    Colloid,  etc. 

(3)  Nuclear  (increase  of  chromatin). 


ii8  ■         POPULAR   SCIENCE   MONTHLY 

known  at  the  present  time.  These  tables  are  taken  from  a  lecture 
which  I  delivered  in  New  York  a  few  years  ago,  which  was  subse- 
quently published.  If  any  of  you  should  care  to  make  a  closer  ac- 
quaintance with  them  they  are  therefore  readily  accessible  to  you.  How 
then,  from  the  standpoint  of  cytomorphosis  ought  we  to  look  upon  old 
age?  Cytomorphosis,  the  succession  of  cellular  changes  which  goes  on 
in  the  body,  is  always  progressive.  It  begins  with  the  earliest  develop- 
ment, continues  through  youth,  is  still  perpetually  occurring  at  maturity 
and  in  old  age.  The  role  of  the  last  stage  of  cytomorphosis,  that  is,  of 
death  in  life,  is  very  important,  and  its  importance  has  only  lately  be- 
come clear  to  us.  I  doubt  very  much  if  the  conception  is  at  all 
familiar  to  the  members  of  this  audience.  iSTevertheless  the  constant 
death  of  cells  is  one  of  the  essential  factors  of  development,  and  much 
of  the  progress  which  our  bodies  have  made  during  the  years  we  have 
lived,  has  been  conditional  upon  the  death  of  cells.  As  we  have  seen, 
cytomorphosis,  when  it  goes  through  to  the  end,  involves  not  only  the 
differentiation  but  the  degeneration  and  death  of  the  parts.  There  are 
many  illustrations  of  this  which  I  might  cite  to  you  as  examples  of  the 
great  importance  of  the  destruction  of  parts.  Thus  there  is  in  the 
embryo  before  any  spinal  column  is  formed  an  actual  structure  which 
is  termed  the  notochord.  In  the  young  mammalian  embryo  this 
structure  is  clearly  present  and  plays  an  important  part,  but  in  the 
adult  it  has  entirely  disappeared,  and  its  disappearance  begins  very 
early  during  embrj^onic  life.  There  are  numerous  blood  vessels  which 
we  find  to  occur  in  the  embryo,  both  those  which  carry  the  blood  away 
from  the  heart  and  those  which  bring  blood  to  the  heart,  which  during 
the  progress  of  development  are  entirely  destroyed,  and  disappear  for- 
ever. Knowledge  of  these  is  to  the  practical  anatomist  and  surgeon 
often  of  great  importance.  A'ast  numbers  of  the  smaller  blood  vessels 
which  we-  know  commonly  by  the  name  of  capillaries,  exist  only,  for  a 
time  and  are  then  destroyed.  There  is  in  the  J-oung  frog,  while  he  is 
in  the  tadpole  stage,  a  kidney-like  organ,  which  on  account  of  its  posi- 
tion is  called  the  head-kidney,  but  it  exists  only  during  the  young 
stage  of  the  tadpole.  There  is  later  produced  another  kidney  which, 
from  its  position,  is  called  the  middle  kidne}^  and  which  is  the  only 
renal  organ  found  in  the  adult,  for  the  head  kidney  entirely  disap- 
pears in  these  animals  long  before  the  adult  condition  is  reached.  In 
the  mammal  there  is  yet  a  third  kindey.  We  have  during  the  em- 
bryonic stage  of  the  mammal  always  a  well-developed  excretory  organ 
which  corresponds  to  the  middle  or  permanent  kidney  of  the  frog,  yet 
during  embryonic  life  the  greater  part  of  this  temporary  structure  is 
entirely  destroyed.  It  is  dissolved  away  and  vanishes,  leaving  only  a 
few  remnants  of  comparatively  little  importance  in  the  adult.  The 
new  structure,  the  permanent  kidney  which  we  have,  takes  its  place 


AGE,    GROWTH   AND    DEATH  119 

functionally.  Large  portions  of  the  tissues,  which  arise  in  the  embryo, 
are  destroyed  at  the  time  of  birth,  and  take  no  share  in  the  subsequent 
development  of  the  child.  If  we  follow  out  with  the  microscope  the 
various  changes  which  go  on  in  the  developing  body  we  see  revealed  to 
us  a  very  large  number  of  cases  of  death  of  tissues,  followed  by  their 
removal.  Thus  the  cartilage  which  exists  in  the  early  stages  dies  and 
is  dissolved  away,  and  its  place  is  taken  by  bone.  Those  things  which 
we  know  as  bony  elements  of  the  skeleton  in  the  adult,  in  the  embryo 
exist  merely  as  cartilage,  but  the  cartilage  is  not  converted  into  bone 
but  it  is  destroyed  and  its  place  taken  by  bone.  There  is  overlying 
the  heart  of  a  child  at  birth  a  well-developed  gland  known  as  the 
thymus.  After  childhood  this  undergoes  a  retrograde  development;  it 
becomes  gradually  absorbed  and  persists  only  in  a  rudimentary  condi- 
tion. With  the  loss  of  the  teeth  occurring  during  infancy,  you  are 
familiar,  and  know  that  the  first  set  of  teeth  are  but  for  a  short  period, 
and  are  to  be  replaced  by  the  permanent  set.  In  very  old  persons  we 
see  a  great  deal  of  the  bony  material  absorbed,  and  this  absorption  of 
the  bone  is  a  phenomenon  which  occurs  at  almost  every  period  of  the 
development.  Portions  of  the  epidermis  or  outer  skin  are  constantly 
shed,  as  is  well  known,  and  the  loss  of  hair  and  the  loss  of  portions  of 
our  nails  are  so  familiar  to  us  that  we  hardly  heed  them.  Of  the 
constant  destruction  of  the  cells,  which  are  found  in  the  lining  of  the 
intestine,  I  have  already  spoken.  At  all  times  in  the  body  there  is  a 
vast  amount  of  destruction  of  blood  corpuscles  going  on,  a  destruction 
v/hich  is  physiologically  indispensable,  for  the  material  which  the  blood 
corpuscles  furnish  is  used  in  many  ways.  For  instance,  the  pigment 
which  occurs  in  the  hair  is  supposed  to  be  derived  from  the  chemical 
substances  the  use  of  which  the  body  obtains  by  destroying  blood 
corpuscles.  One  of  the  most  familiar  instances  of  destruction  is  that 
of  the  tail  of  the  tadpole.  The  young  frog  and  the  young  toad  during 
their  larval  stages  live  in  the  water  and  each  of  them  is  furnished 
with  a  nice  tail  for  swimming  purposes.  As  the  time  approaches  for 
the  metamorphosis  of  the  tadpole  into  the  adult,  the  tail  is  gradually 
dissolved  away.  It  is  not  cast  off,  but  it  is  literally  dissolved,  resorbed, 
and  vanishes  ultimately  altogether. 

It  is  evident  that  such  a  vast  amount  of  destruction  of  living  cella 
could  not  be  maintained  in  the  body  without  the  body  going  entirely 
to  destruction  itself,  were  there  not  some  device  for  making  good  the 
losses  which  are  thus  l)rought  al)Out.  We  find  in  fact  that  there  is 
always  a  reserve  of  cells  kept  to  make  good  the  loss  which  it  is  essential 
should  be  made  good.  Some  losses  apparently  do  not  have  to  be  re- 
paired, but  the  majority  of  them  must  be  compensated  for,  and  thia 
is  done  by  having  in  the  body  a  reserve  supply  of  cells  which  can 
produce  new  cells  of  the  sort  required.  This  leads  us  to  considera- 
tion of  the  phenomenon  of  regeneration   and  of  the  repair  of  parts. 


I20  POPULAR   SCIENCE   MONTHLY 

These  phenomena  we  can  better  take  up  later  in  onr  course,  when  we 
have  dealt  with  the  general  processes  of  development  and  growth. 
From  the  study  of  regeneration  we  shall  be  able  to  confirm  the  explana- 
tion of  old  age,  which  I  want  to  lay  before  you.  This  confirmation  is 
so  important  that  it  will  be  better  taken  up  in  a  separate  lecture,  than 
slipped  in  now  when  the  hour  is  nearly  by. 

Old  age,  after  what  I  have  said,  I  think  you  will  all  recognize  as 
merely  the  advanced  and  final  stage  of  cytomorphosis.  Old  age  differs 
but  little  in  its  cytomorphosis  from  maturity;  maturity  differs  much 
from  infancy ;  infancy  differs  very  much  indeed  from  the  embryo ;  but 
the  embryo  differs  enormously  from  the  germ  in  its  cytomorphic  con- 
stitution. We  know  that  in  the  early  time  comes  the  great  change, 
and  this  fact  we  shall  apply  for  purposes  of  interpretation  later  on. 
Cytomorphosis  is  then  a  fundamental  notion.  It  gives  us  in  a  general 
law,  a  comprehensive  statement  of  all  the  changes  which  occur  in  the 
body.  None,  in  fact,  are  produced  at  any  period  in  any  of  us  except 
in  accordance  with  this  general  cytomorphic  law.  There  is,  first,  the 
undifferentiated  stage,  then  the  progressive  difl'erentiation ;  next  there 
follows  the  degenerative  change  ending  in  death,  and  last  of  all  the 
removal  of  the  dead  cells.  Such  we  may  conveniently  designate  as  the 
four  essential  stages  of  cytomorphosis.  This  cytomorphosis  is  at  first 
very  rapid;  afterwards  it  becomes  slower.  That  is  a  significant  thing. 
The  young  change  fast ;  the  old  change  slowly.  We  shall  be  able,  when 
we  get  a  little  farther  along  in  our  study,  to  see  that  in  differentiation 
lies  the  explanation  of  a  great  deal  of  biological  knowledge,  lies  the 
explanation  of  our  conception  of  cell  structures ;  and  in  it  also  lies  not 
only  the  explanation  of  the  death  of  cells,  but  also,  as  it  seems  to  me — 
and  this  is  one  of  the  points  that  I  shall  want  particularly  to  bring 
forward  before  the  close  of  the  course — of  general  death,  that  which 
we  mean  by  death  in  common  parlance,  when  the  continuation  of  the 
life  of  the  individual  ceases,  and  is  thereafter  bodily  impossible.  The 
explanation  of  death  is  one  of  the  points  at  which  we  shall  be  aiming 
in  the  subsequent  lectures  of  the  course.  Now  we  know  that  in  con- 
nection with  age  there  is  always  growth.  I  propose,  therefore,  in  the 
next  lecture,  to  take  up  the  subject  of  growth.  We  shall  arrive  at 
some  paradoxical  conclusions,  for  it  can  be  shown  by  merely  statistical 
reckonings  that  our  notion  that  man  passes  through  a  period  of  de- 
velopment and  a  period  of  decline  is  misleading,  in  that  in  reality  we 
begin  with  a  period  of  extremely  rapid  decline,  and  then  end  life  with 
a  decline  which  is  very  slow  and  very  slight.  The  period  of  most 
rapid  decline  is  youth;  the  period  of  slowest  decline  is  old  age,  and 
that  this  statement  is  correct  I  shall  hope  to  prove  to  3^ou  with  the 
aid  of  tables  and  lantern  illustrations  at  the  next  lecture. 


THE 

POPULAR    SCIENCE 

MONTHLY. 


SEPTEMBER,  1907 


THE  PROBLEM  OF  AGE,  GROWTH  AND  DEATH 

By  CHARLES  SEDGWICK  MINOT,  LL.D.,  D.Sc. 

JAMES  STILLMAN   PROFESSOR  OF  COMPARATIVE  ANATOMY   IN  THE 
HARVARD  MEDICAL  SCHOOL 

III.  The  Rate  of  Growth 

Ladies  and  Gentlemen:  In  the  first  of  the  lectures,  I  described  those 
grosser  characteristics  of  old  age,  which  we  ourselves  can  readily  dis- 
tinguish, or  which  an  anatomical  study  of  the  body  reveals  to  us.  In 
the  second  lecture  I  spoke  of  the  microscopic  alterations  which  occur 
in  the  body  as  it  changes  from  youth  to  old  age.  But  besides  the 
changes,  which  we  have  already  reviewed,  there  are  those  others,  very 
conspicuous  and  somewhat  known  to  us  all,  which  we  gather  together 
under  the  comprehensive  term  of  growth.  It  is  grt)wth  which  I  shall 
ask  you  to  study  with  me  this  evening,  and  I  shall  hope,  by  the  aid 
of  our  study,  to  reinforce  in  your  minds  the  conclusion  which  I  have 
already  indicated,  that  the  early  period  of  life  is  a  period  of  rapid 
decline,  and  that  the  late  period  of  life  is  one  of  slow  decline. 

In  order  to  study  growth  accurately,  it  is  desirable,  of  course,  to 
measure  it,  but  since  we  are  concerned  with  the  general  problem  of 
growth,  we  wish  no  partial  measure,  such  as  that  of  the  height  alone 
would  be.  And  indeed,  if  we  take  any  such  partial  measure,  how 
could  we  compare  different  forms  with  one  another?  The  height  of  a 
horse  is  not  comparable  to  that  of  a  man;  the  height  of  a  caterpillar 
is  not  comparable  to  that  of  any  vertebrate.  Katurally,  therefore,  we 
take  to  measuring  the  weight,  which  represents  the  total  mass  of  the 
living  body,  and  enables  us  at  least  with  some  degree  of  accuracy  to 
compare  animals  of  different  sorts  with  one  another.  Now  in  studying 
this  question  of  the  increase  of  weight  in  animals,  as  their  age  in- 
creases, it  is  obviously  desirable  to  eliminate  from  our  experiments  all 
disturbing  factors  which  might  affect  the  rate  of  growth  or  cause  it 
to  assume  irregularities  which  are  not  inherent  either  in  the  organiza- 


194 


POPULAR    SCIENCE   MONTHLY 


tion  of  the  animal  or  in  the  changes  age  produces.  The  animals  which 
belong  to  the  vertebrate  sub-kingdom,  of  which  we  ourselves  are  mem- 
bers, can  be  grouped  in  two  large  divisions  according  to  the  natural 
temperature  of  their  bodies.  The  lower  vertebrates,  the  fishes,  frogs 
and  their  kin,  are  animals  which  depend  for  their  body  temperature 
more  or  less  on  the  medium  in  which  they  live.  The  other  division 
of  vertebrate  animals,  which  includes  all  the  higher  forms,  are  so 
organized  that  they  have  within  certain  limits  the  power  of  regulating 
their  own  body  temperature.  Now  it  is  easily  to  be  observed — and 
any  one  who  has  made  observations  upon  the  growth  of  animals  can 
confirm  this — that  animals  otherwise  alike  will  grow  at  different  speeds 

at  different  temperatures. 
There  are  animals,  like  the 
frogs  and  salamanders, 
which  will  live  at  a  very 
considerable  range  of  tem- 
perature and  thrive,  ap- 
parently. No  ultimate  in- 
Jury  is  done  to  them  by  a 
change  of  their  bodily  tem- 
perature. Here  we  have  a 
picture  of  four  young  tad- 
poles, all  of  which  are  ex- 
actly three  days  old.  The 
first  of  these  has  been  kept 
at  a  temperature  not  much 
above  freezing.  The  fourth, 
at  a  temperature  of  about 
24  degrees  centigrade;  the 
other   two   at  temperatures 

Pig.  19.    Four  Tadpoles  of  the  Eukopean  Prog,  ,     ,  rr>v,                    n    A 

Rana  fusca.    After  OskarHertwig.    The  four  animals  between.  i  hey    are    aii    QC- 

are  all  of  the  same  age  (three  days)  and  raised  from  the  sCCndants  from     the     Same 

same  batch  of  eggs,  but  have  been  kept  at  different  tern-  ,     ,    n          «  ^          ,                          -, 

peratares.  oatch    of    frogs    cggs,    and 

yl  at  11.5°  Centigrade.  £  at  15.0°  Centigrade.  you     Can     SCC     readily     that 

C  "  20.0°  "  D  "  24.0°  "  • .  n      ,  •  ,-n 

the  first  one  is  still 
essentially  nothing  but  an  egg.  The  second  one,  which  has  had  a 
little  higher  temperature,  already  shows  some  traces  of  organization, 
and  those  familiar  with  the  development  of  these  animals  can  see  in 
the  markings  upon  the  surface  the  first  indications  of  the  differentia- 
tion of  the  nervous  system.  The  third  has  been  kept  at  a  considerably 
warmer  temperature,  and  is  now  obviously  a  young  tadpole;  here  are 
the  eyes,  the  rudimentary  gills,  the  tail,  etc.  While  the  fourth  tadpole, 
which  was  maintained  at  the  best  temperature  for  the  growth  of 
these  animals,  has  advanced  enormously  in  its  development.  Obviously, 
should  we  make  .experiments  upon  animals  of  this  class  it  would  be 


AGE,    GROWTH   AND    DEATH 


195 


necessary  to  keep  them  at  a  uniform  temperature,  if  we  wished  to 
study  their  rate  of  development,  and  that  is,  for  very  practical  reasons, 
extremely  difficult  and  unsatisfactory.  Far  better  it  has  seemed  for 
our  study  of  growth  to  turn  to  those  animals  which  regulate  their  own 
temperature.  This,  accordingly,  I  have  done,  and  the  animal  chosen 
for  these  studies  was  the  guinea-pig,  a  creature  which  offers  for  such 
investigations  certain  definite  advantages.  It  is  easily  kept;  it  is  apt 
to  remain,  with  proper  care,  in  good  health.     Its  food  is  obtainable  at 


(/yi, 
64 

63 

50 

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56 

54 

52 

50 

48 

46 

44 

42 

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Uo. 

140 
ISO 
120 
110 
100 
90 
80 
70 
60 
50 
40 

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Ad   '^       5        7       8        9       10       II       12      13       14      15      16       17       IB      1 

3 

Fig.  20.    Curves  showing  the  Growth  cf 
AND  Weight.    Alter 


Boston  School  Children  in  Height 
H.  P.  Bowditch. 


all  seasons  of  the  year,  in  great  abundance,  and  at  small  expense.  The 
animals  themselves  being  of  moderate  size  do  not,  of  course,  require 
such  extraordinary  amounts  of  food  as  the  large  animals,  should  we 
experiment  with  them.  Accordingly  with  guinea-pigs  I  began  making, 
years  ago,  a  long  series  of  records,  taking  from  day  to  day,  later  from 
week  to  week,  and  then,  as  the  animals  grew  older,  month  by  month, 
the  weight  of  recorded  individuals.  There  was  thus  obtained  a  body 
of  statistics  which  rendered  it  possible  to  form  some  idea  of  the  rapidity 
of  growth  of  this  species  of  mammal. 

Now  in  regard  to  the  rapidity  of  growth,  it  is  necessary  that  we 
form  clearer  notions  than  perhaps  you  started  out  with  when  you  came 
into  the  hall  this  evening.  I  will  ask  for  the  next  of  our  pictures 
on  the  screen,  where  we  shall  see  illustrated  to  us  older  methods  of 


196  POPULAR    SCIENCE   MONTHLY 

recording  the  progressive  growth  of  animals.  This  is  a  chart  taken 
from  the  records  of  my  friend,  Dr.  Henry  P.  Bowditch,  showing  the 
growth  of  school  children  in  Boston.  Here  we  have,  in  the  lower  part 
of  the  figure,  the  two  curves  of  growth  in  weight.  The  upper  curve 
if;  the  weight  of  boys.  We  can  follow  it  back  through  the  succession 
of  years  down  to  the  age  of  five  and  one  half  years,  when  the  records 
begin.  The  child  weighs,  as  you  see,  a  little  over  forty  pounds  at  that 
time.  When  the  boy  reaches  the  age  of  eighteen  and  one  half  years,  he 
approaches  the  adult  size,  and  weighs  well  over  130  pounds.  Here  then 
we  see  growth  represented  to  us  in  the  old  way,  the  progressive  in- 
crease of  the  animal  as  it  goes  along  through  the  succession  of  years. 
Kow  this  is  a  way  which  records  the  actual  facts  satisfactorily.  It 
shows  the  progressive  changes  of  weight  as  they  really  occur;  but  it 
does  not  give  us  a  correct  impression  of  the  rate  of  growth.  Concern- 
ing the  rate  of  growth,  some  more  definite  notion  must  be  established  in 
our  minds  before  we  can  be  said  to  have  an  adequate  conception  of 
the  meaning  of  that  term.  It  is  from  the  study  of  the  statistics  of 
the  guinea-pigs,  and  of  other  animals,  which  I  have  since  had  an 
opportunity  of  experimenting  with,  that  we  get  indeed  a  clearer  insight 
as  to  what  the  rate  of  growth  really  is  and  really  means. 

I  should  like  to  pause  a  moment  to  say  that  when  I  first  published 
a  paper  upon  the  subject  of  growth,  it,  fortunately  for  me,  interested 
the  late  Dr.  Benjamin  Gould.  The  experiments  which  I  had  made  and 
recorded  in  that  first  publication  came  to  a  sudden  end,  owing  to  a 
disaster  for  which  I  myself  was  personally  not  responsible,  by  which 
practically  my  entire  stock  of  animals  was  suddenly  destroyed.  Dr. 
Gould,  after  consulting  with  me,  proposed  that  I  should  have  further 
aid  from  the  National  Academy  of  Sciences,  and  through  his  inter- 
vention I  obtained  a  grant  from  the  Bache  fund  of  the  academy.  That 
liberal  grant  enabled  me  to  continue  these  researches,  and  this  is  the 
first  comprehensive  presentation  of  my  results  which  I  have  attempted. 
In  this  and  the  subsequent  lectures,  I  hope  that  enough  of  what  is  new 
in  scientific  conclusions  may  appear  to  make  those  to  whose  generosity 
1  am  indebted  feel  that  it  has  been  worthily  applied.  I  can  not  let 
such  an  occasion  as  this  pass  by  without  expressing  publicly  my 
gratitude  to  Dr.  Gould  for  his  encouragement  and  support  at  a  time 
when  I  most  keenly  appreciated  it. 

If  animals  grow,  that  which  grows  is  of  course  the  actual  substance 
of  the  animal.  Now  we  might  say  that  given  so  much  substance  there 
should  be  equal  speed  of  growth,  and  we  should  expect,  possibly,  to 
find  that  the  speed  would  be  more  or  less  constant.  I  can  perhaps 
illustrate  my  meaning  more  clearly,  and  briefly  render  it  distinct  in 
your  minds,  by  saying  that  if  the  rate  of  growth,  as  I  conceive  it,  should 
remain  constant,  it  would  take  an  animal  at  every  age  just  the  same, 
length  of  time  to  add  ten  per  cent,  to  its  weight;  it  would  not  be  a 


AGE,    GROWTH   AND    DEATH 


197 


question  whether  a  baby  grew  an  ounce  in  a  certain  length  of  time, 
and  a  boy  a  pound  in  tlie  same  time,  for  the  pound  might  not  be 
the  same  percentage  of  advance  to  the  boy  that  the  ounce  would  be  to 
the  baby.  In  reality  with  an  advance  of  an  ounce  the  baby  might  be 
growing  faster  than  the  older  boy  with  the  addition  of  the  pound. 

In  the  next  slide  which  we  are  to  have  thrown  upon  the  screen 
we  have  my  method  of  measuring  rate  of  growth  illustrated  graphically. 
There  is  here  a  curve  which  represents  the  rate  of  growth  of 
male  guinea-pigs.  The  figures  at  the  bottom  indicate  the  age  of  the 
animals  in  days.  Wlien  guinea-pigs  are  born,  they  are  very  far  ad- 
vanced in  development,  and  the  act  of  birth  seems  to  be  a  physiological 


25811     n    23   29  J5J8  45  60  75  90  IC3  120  135  150  l65  16 


Fig.  21.    Curve  showing  the  Daily  Percentage  Increments  in  Weight  of 
JIale  Guinea  Pigs. 


shock  from  which  the  organism  suffers,  and  there  is  a  lessening  of  the 
power  of  growth  immediately  after  birth.  But  in  two  or  three  days 
the  young  are  fully  recovered,  and  after  that  restoration  they  can  add 
over  five  per  cent,  to  their  weight  in  a  single  day.  But  by  the  time 
they  are  17  days  old,  as  represented  by  this  line,  they  can  add 
only  four  per  cent.,  and  by  the  time  they  are  24  days  old,  less  than 
two  per  cent. ;  at  45  barely  over  one  per  cent. ;  at  70  still  over  one 
per  cent.;  at  90  less;  at  160  less;  and  towards  the  end  the  curve  con- 
tinues dropping  off,  coming  gradually  nearer  and  nearer  to  zero,  to 
which  it  closely  approximates  at  the  age  of  240  days.  In  about  a  year, 
the  guinea-pig  attains  nearly  its  full  size.  You  notice  that  this  curve 
is  somewhat  irregular.  Such  is  very  apt  to  be  the  result  from  statistics 
when  the  number  of  observations  is  not  very  large.  It  means  simply 
that  there  was  not  a  sufficiently  large  number  of  animals  measured  to 
give  an  absolutely  even  and  regular  set  of  averages.  But  the  general 
course  of  the  curve  is  very  instructive.  In  the  earlier  condition  of  the 
young  guinea-pig  there  is  a  rapid  decline;  in  the  later,  a  slow  decline. 


198 


POPULAR    SCIENCE   MONTHLY 


'J'he  change  from  rapid  to  slow  decline  is  not  sudden,  but  gradual,  as 
you  see  by  the  general  character  of  this  curve. 

In  the  next  slide  we  can  see  immediately  that  what  I  have  asserted 
as  true  of  the  male  is  equally  true  of  the  female,  although  the  values 


?.  5811     17    23    29  3S3e   45 


105  120  135  ISO  165  180 


210  dai^  241 


Fig.  22.    Curve  showing  the  Daily  Percentage  Increments  in  Weight  of 
Female  Guinea-pigs. 

which  we  have  differ  slightly  in  the  two  sexes,  and  there  are  accidental 
but  not  significant  variations  in  this  curve  as  in  the  first.  Here  also 
we  observe  at  once  an  early  period  of  rapid  decline  in  which  the  rate 
of  growth  is  going  down  and  down — a  period  of  slight  decline  in  which, 
to  be  sure,  it  is  going  down  still,  but  with  diminished  rapidity. 

There  is  another  method  by  which  we  can  represent  this  change 


da/ua 
50 


40 


30 


20 


X-emyot^  o^  fO%  J^e^Uocid.    TYlaie^. 


f^eA^UTctZ    4    5    6     7    8    9    10    II    12    13    14   15   16   17   18    19  20  21   21  23  24  25 


Length  of  Time  required  to  make  Each  Successive 
Increase  of  10  per  cent,  in  Weight  by  Male  Guinea-pigs. 


AGE,    GROWTH    AND    DEATH 


199 


in  the  rate  of  growth  which  will  perhaps  help  to  illustrate  it;  and  in 
the  next  of  our  pictures  we  see  this  other  form  of  representation  befora 
us.  This  vertical  line  represents  the  length  of  time  which  it  takes  a 
young  male  guinea-pig  to  add  ten  per  cent,  to  its  weight  the  first  time. 
Here  the  third  time— the  fourth— the  fifth— and  you  see  as  it  is  grow- 
ing older  and  older  it  takes  the  animal  longer  and  longer  to  add  ten 
per  cent,  to  its  weight.  Finally  we  get  to  the  nineteenth  addition,  and 
we  see  that  the  period  is  very  long  indeed.  How  long  that  period  is 
we  can  judge  by  the  figures  here  upon  the  left,  which  represent  the 
length  of  the  days.  From  the  base  line  to  this  one  marked  "  ten  "  is  a 
period  of  ten  days,  and  you  see  from  the  time  the  guinea-pig  has  added 
to  its  weight  ten  per  cent,  for  the  nineteenth  time  it  does  it  so  slowly 
that  it  requires  ten  days  and  more;  for  the  twenty-first  time,  nearly 
twenty;  for  the  twenty-second  time,  nearly  forty.  Here  where  the 
number  of  observations  becomes  small,  the  curve  grows  very  irregular. 
Thus  we  demonstrate  that  as  the  animal  grows  older  it  takes  longer 
and  longer  to  add  ten  per  cent,  to  its  weight.  In  the  other  sex,  as  the 
next  slide  shows,  the  same  phenomena  can  be  clearly  demonstrated; 
here  are  the  periods  as  before,  lengthening  out,  as  you  see,  at  first; 
then  becoming  very  long  indeed.     In  the  following  slide  I  have  another 


oUi/u<s 


60 


50 


40 


30 


20 


10 


X.(y>vqi/i'  ai  W%  Pe/ucvU    3'e/>-yi42l&<i . 


or>-T~T 

J^eA^^jzCZ    4    5    6    7    8    9    10   II    12   13    14  15    16    l7   18    19  2Q  21  Zl  23  24  25 

FlO.  24.     CUUVE  SHOWING  THE  LENGTH   OF  TiME   RFXiUIRED  TO  MAKE  EACH  SUCCESSIVE 
INCKEAHE  OF  10  PER  CENT.   IN   WEIGHT   BY   FEMALE  GUINEA-l'IGS. 


200  POPULAR    SCIENCE   MONTHLY 

form  of  representation  of  this  same  phenomenon  as  it  occurs  in  the 
human  subject.  Here  is  a  diagram  of  growth,  which  represents,  as 
accurately  as  I  could  determine  it,  the  curve  complete  for  man  from 
the  date  of  birth  up  to  the  age  of  forty  years.  It  has  been  calculated 
by  a  simple  mathematical  process  where  these  ten-per-cent.  increments 
fall,  and  from  each  point  in  this  curve  where  there  has  been  such  an 
increment,  a  vertical  line  has  been  drawn,  as  you  see  here.  These  lines 
are  very  close  together  at  the  start.  One  ten  per  cent,  after  another 
follows  in  a  short  interval  of  time,  but  gradually  the  time,  as  indicated 
by  the  space  between  two  of  these  vertical  lines,  increases,  and  when  the 
individual  is  three  years  old,  you  can  see  there  has  been  a  very  great 


2^''              ^^{''^               iiyrs 

30               125.02  lbs. 

iZO 

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60 

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20              48.20  lbs. 

/ 

/ 

40 

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15 

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20 

10      A 

x 

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11 

Fig.  25.    Curve  showing  the  Growth  of  Man  from  Birth  to^Maturity,  with  vertica 
Uaes  added  to  mark  the  duration  ot  the  periods,  for  each  10  per  cent,  addition  to  the  weight. 

lengthening  out  of  the  period  which  is  necessary  for  it  to  add  ten  per 
cent,  to  its  weight.  Then  it  comes  at  the  age  of  twelve  to  a  period  of 
slightly  more  rapid  growth,  a  fluctuation  which  is  characteristic  of 
man,  but  does  not  appear  in  the  majority  of  animals.  After  that 
comes  very  rapidly  the  enormous  lengthening  of  the  period;  and  I  have 
not  added  the  last  ten  per  cent,  because  the  curve  here  at  the  top,  you 
see,  is  not  very  regular,  and  it  could  not  be  calculated  with  certainty. 
Our  diagram  is  merely  another  form  of  graphic  representation  of  the 
fact  that  the  older  we  are  the  longer  it  takes  us  to  grow  a  definite 
proportional  amount. 

The  next  slide  carries  us  into  another  part  of  our  study,  away  from 
the  mammals  which  we  have  thus  far  considered,  into  the  class  of 
birds.  The  growth  of  chickens  is  represented  here.  Now  a  chicken  is 
born  in  a  less  matured  state  than  a  guinea-pig,  and  has  a  good  deal 


AGE,    GROWTH   AND    DEATH 


20I 


03f8  13  i82i  283338  «    S6    66     '7       90        '06 


Fig.  26J    Curve  showing  the  Daily  Percentage  Increments  in  Weight 
BY  Male  Chickens. 

higher  efficiency  of  growth  at  first.  In  a  chicken,  as  in  a  guinea-pig, 
birth  is  a  disturbing  factor,  and  growth  immediately  after  the  hatching 
of  the  chicken  is  a  little  impeded,  but  the  chick  quickly  recovers  and, 
as  we  see,  the  first  time  when  the  rate  can  be  distinctly  measured  we  get 
a  nine-per-cent.  addition  to  the  weight  in  a  single  day.  In  a  chicken 
as  in  the  guinea-pig,  the  rate  gradually  diminishes.  The  change  from 
the  rapid  decline  at  first  to  the  later  slower  decline  is  more  gradual ;  the 
curve  is  more  distinctly  marked  in  the  chicken  as  a  round  curve.  There 
is  not  in  the  bird  so  marked  a  separation  of  the  preliminary  rapid  de- 
cline and  the  later  slower  decline  as  we  find  in  the  guinea-pig.  The 
curve  again  is  very  irregular  because  I  had  only  a  very  limited  number 
of  observations  upon  the  weight  of  chicks.  The  other  sex,  as  the  next 
slide  will  show,  presents  similar  phenomena,  though  the  female  chickens 
do  not  grow  quite  as  fast  as  their  brothers.     Here  we  notice  an  increase 


■^1 

A 

PeAcsm^yt^  J-naU'rm/n/a       CAccA^     •J'e'ryiaAd 

\ 

m 

\ 

\^ 

\  _ 

y 

/ 

\ 

N 

-^ 

^ 

~~ ■ — r 

1 

OMa  1313222833^6  4«    56    66     '-      90       106 


laUi^j^ 


Fig.  27.    CuEVK  showing  the  daily  Percentage  Increments  in  Weight 
BY  Female  Chickens. 


202  POPULAR   SCIENCE   MONTHLY 

of  almost,  but  not  quite  nine  per  cent.,  rapidly  falling  down  so  that 
after  the  chick  is  two  months  old  it  never  adds  as  much  as  three  per 
cent,  to  its  weight.  It  loses  in  the  first  two  months  from  a  capacity  to 
add  nine  per  cent.,  down  to  a  capacity  of  adding  less  than  three.  It 
loses  in  two  months  two  thirds  of  its  total  power  of  growth,  for  from 
nine  to  zero  is  divisible  into  two  parts,  of  which  the  first,  from  nine 
down  to  three,  would  be  two  thirds,  and  the  second,  from  three  to  zero, 
would  be  one  third.  Here  then  we  learn  that  two  thirds  of  the  decline 
which  occurs  in  the  life  of  a  chick  takes  place  in  two  months,  and  for  the 
rest  of  the  life  of  the  bird  there  is  a  decline  of  one  third.  That,  you 
must  acknowledge,  is  an  extraordinary  and  most  impressive  difference. 
If  it  be  true  that  the  more  rapid  growth  depends  upon  the  youth  of  the 
individual, — its  small  distance  in  time  from  its  procreation,  then  we 
may  perhaps,  by  turning  to  other  animals  which  are  born  in  a  more 
immature  state,  get  some  further  insight  into  these  changes;  and  that 
I  have  attempted  to  do  by  my  observations  upon  the  development  of 
rabbits,  Eabbits,  as  you  know,  are  born  in  an  exceedingly  immature 
state.  They  are  blind,  they  are  naked,  they  are  almost  incapable  of 
definite  movements,  quite  incapable  of  locomotion,  and  are  hardly  more 
than  little  imperfect  creatures  lying  in  the  nest  and  dependent  utterly 
upon  the  care  of  the  mother,  quite  unable  to  do  anything  for  them- 
selves except  take  the  milk  which  is  their  nourishment.  They  are  in- 
deed animals  born  in  a  much  less  advanced  stage  than  are  the  guinea- 
pigs.  Upon  the  screen  we  see  this  interesting  result  demonstrated  to 
us,  that  a  male  rabbit,  the  fourth  day  after  its  birth,  is  able  to  add  over 
seventeen  per  cent,  to  its  weight  in  one  day.  From  that  the  curve 
drops  down,  as  you  see,  with  amazing  rapidity,  so  that  here  at  an  age 
of  twenty-three  days  the  rabbit  is  no  longer  able  to  add  nearly  eighteen 
per  cent,  daily,  but  only  a  little  over  six.  At  the  end  of  two  months 
from  its  birth,  the  growth  power  of  the  rabbit  has  dropped  to  less  than 
two  per  cent.,  and  at  two  months  and  a  half  it  has  dropped  to  one.  The 
drop  in  two  and  a  half  months  has  been  from  nearly  eighteen  per  cent, 
down  to  one  per  cent.,  and  the  rest  of  the  loss  of  one  per  cent,  is 
extended  over  the  remaining  growing  period  of  the  rabbit.  Could  we 
have  a  more  definite  and  certain  demonstration  of  the  fact  that  the 
decline  is  most  rapid  in  the  young,  most  slow  in  the  old?  It  is  not 
in  this  case  any  more  than  in  the  others  the  one  sex  that  demonstrates 
this  fact,  for  in  the  female  we  find  exactly  the  same  phenomena,  as  the 
next  slide  will  show.  The  irregularities  are  not  significant.  The 
strange  dip  at  thirty-eight  days,  for  instance,  corresponds  to  an  illness 
of  some  of  the  rabbits  which  were  measured,  but  they  rapidly  recovered 
from  it  and  grew  up  to  be  fine,  nice  rabbits.  If  instead  of  measuring 
half  a  dozen  rabbits,  we  had  measured  two  hundred  or  five  hundred, 
these  irregularities  would  certainly  have  disappeared.  The  females  in 
the  case  of  the  rabbits,  as  in  the  case  of  the  guinea-pigs,  are  not  able 


AGE,    GROWTH   AND   DEATH 


203 


PcAce/nJcu^  Jnot^e^ne^o^ytj    J2aMv^ 


rriaA^ 


03  8  13 1833  ?8  33  38       55  77| 


1O64 


iSOoizy^ 


Fig.  28.     CCRVE  SHOWING  THE  DAILY  PERCENTAGE  INCKEMENTS  IN  WEIGHT 

BY  Male  Rabbits. 

to  grow  quite  so  fast  at  first.  We  see  here  sixteen  instead  of  over  seven- 
teen per  cent,  as  the  initial  value,  but  the  general  character  of  the  drop 
is  the  same,  enormously  rapid  at  first  and  very  slow  afterwards.  All 
of  our  cases,  then,  show  the  same  fundamental  phenomena  appearing 
with  different  values. 

Xow  in  regard  to  man,  we  do  not  possess  any  such  adequate  series 
of  statistics  of  growth  as  is  desirable.  We  have  many  records  of  tlie 
weight  of  babies,  by  which  I  mean  children  from  the  date  of  birth  up 
to  one  year  of  age.  We  have  also  very  numerous  records  of  school 
children,  which  will  extend  perhaps  from  five  and  one  half  up  to  say 
seventeen,  eighteen  or  even  nineteen  years.     There  are  records  of  boys 


2  04 


POPULAR   SCIENCE  MONTHLY 


Fig.  29.    Curve  showing  the  Daily  Peecentagk  Increments  in  Weight 
BY  Female  Rabbits. 


at  universities,  and  a  still  more  limited  number  of  weighings  of  girls 
at  colleges.  But  all  these  statistics  piled  together  do  not  give  us  one 
comprehensive  set  of  data  including  all  ages.  This  is  very  much  to  be 
regretted,  and  it  would  be  an  important  addition  to  our  scientific  knowl- 
edge could  statistics  of  the  growth  of  man  be  gathered  with  due  precau- 
tions. It  would  fill  one  of  the  gaps  in  our  knowledge  which  is  lament- 
able. We  have,  however,  some  rough,  imperfect  data  which  for  our 
present  purposes  it  seems  to  me  are  adequate,  and  the  results  of  the 
study  of  these  will  be  shown  by  the  next  series  of  pictures. 

But  let  us  pause  for  a  moment  to  consider  this  singular  table.     It 
shows  in  this  column  the  number  of  days  which  it  takes  for  each  species  ^ 


AGE,    GROWTH   AND   DEATH 


205 


Table^ 


Days  Needed  to 
Double  Weight 

100  Parts  Mother's  Milk  Contain 

Species 

Proteid 

Ash 

Lime 

Phosphoric 
Acid 

Man 
Horse 
Cow 
Goat 

Pig 

Sheep 

Cat 

Dog 

Eabbit 

180 
60 
47 
19 
18 
10 

9^ 

8 

7 

1.6 
2.0 
3.5 
4.3 
5.9 
6.5 
7.0 
7.3 
10.4 

0.2 
0.4 
0.7 
0.8 

0.9 
1.0 
1.3 
2.4 

0.0328 
0.124 
0.160 
0.210 

0.272 

0.453 
0.8914 

0.0473 
0.131 
0.197 
0.322 

0.412 

0.493 
0.9967 

of  animal  indicated  at  the  left  to  double  its  weight  after  birth.  A 
man  requires  180  days  to  double  his  weight;  a  horse,  60;  a  cow,  47; 
a  goat,  19;  a  pig,  18;  a  sheep,  10;  a  cat,  9I/2;  a  dog,  8;  a  rabbit,  6  (or 
possibly  7  days).  Now  here  are  analyses  of  the  milk.  The  main 
point  of  interest  is  to  be  found  in  the  figures  in  this  column,  which 
represent  the  amount  of  albuminoid,  or  proteid  material  contained  in 
the  milk.  You  will  observe  that  for  man  the  proportion  is  lowest,  1.6 
per  hundred  parts;  the  horse  has  a  little  more — 2;  cattle — 3.5; 
and  so  the  values  run.  In  other  words,  it  is  obvious  that  the  less  the 
proteid  in  the  milk,  the  longer  does  the  species  require  to  double  its 
weight.  This  looks  at  first  sight  as  if  there  were  a  relation  between 
the  composition  of  the  milk  and  the  period  of  growth  of  the  animal; 
but  you  know  very  well  that  if  you  take  the  milk  of  a  cow,  which  is 
very  much  richer  in  proteid  material,  and  feed  it  to  a  baby,  a  human 
baby,  that  baby  does  not  grow  at  the  same  rate  as  the  young  cow,  but 
grows  at  the  human  rate.  It  is  obvious,  therefore,  that  it  is  somewhat 
more  complicated  than  a  mere  question  of  food  supply.  We  have  in 
fact  one  of  the  beautiful  illustrations  of  the  teleological  mechanism  of 
the  body.  These  various  species  have  their  characteristic  rates  of  growth, 
and  by  an  exquisite  adaptation,  the  composition  of  the  mother's  milk 
has  become  such  that  it  supplies  the  young  of  the  species  each  with  the 
proper  quantum  of  proteid  material  which  is  needed  for  the  rate  of 
growth  that  the  young  offspring  is  capable  of.  It  is  a  beautiful  adjust- 
ment, but  there  is  not  a  causal  relation  between  proteid  matter  and  this 
rate  of  growth.     It  is  an  example  of  correlation,  not  of  causation. 

We  pass  now  to  the  next  of  our  slides,  which  carries  us  over  into 
the  study  of  our  own  species.  It  is  not  possible  at  the  present  time  to 
represent  in  any  form  of  curve  which  I  have  seen  the  daily  percentages 
of  increment  for  man  covering  the  whole  period  of  growth.  In  order 
to  get  the  results  together,  I  have  confined  myself  here  to  the  repre- 
sentation of  the  yearly  percentages.  Now  from  the  age  of  zero  to  the 
age  of  one  year,  you  see  according  to  this  table  a  child  is  able  to  in- 
crease its  weight  200  per  cent.     But  from  the  beginning  of  the  first  to 

*  After  Abdcrhalden,  Zeitschrift  fiir  Physiologische  Chemie,  Band  XXVI., 
p.  497. 


2o6  POPULAR   SCIENCE   MONTHLY 


200% 


[00% 


20% 
10% 


^"^y_^^_^                           / ^ 

\  1  1  r        ^tt>--^ 

YEfiRS     1       2       3      4       5      6       7      8       9      10     11       12      13      14      15      16      17      18     19     20    21     22     23     24    25 

Fig.  30. 


20p% 


20% 
10% 


\ 


Je<zAM  ■■^:M/:j^n^a^  ^..e^nc4£^fzen^. 


0^^&t.^.»:Qcm.2Ui.cn^. 


VEARS     I       2       3       4       S       6       7       8       9      10      a       12      13      14      15      16      17      IS      13      20    21      22     23     24 

Fig.  31. 


AGE,    GROWTH   AND    DEATH 


207 


the  end  of  the  second  year,  only  20  per  cent.,  and  thereafter  it  fluctuates 
in  the  neighborhood  of  10  per  cent,  a  year  until  the  age  of  13.  At  14 
or  15  there  is  a  fluctuation,  an  increase,  and  then  the  decline  goes  on 
again  and  slowly  we  see  the  growth  power  fading  out.  Authors  are  not 
agreed  as  to  the  exact  statistical  value,  and  so  I  will  ask  to  have  thrown 
upon  the  screen  another  curve,  also  representing  the  percentage  increase 
of  boys,  and  based  chiefly  upon  English  tables.  For  these  data  I  am 
indebted  to  my  friend  Professor  Donaldson,  of  the  Wistar  Institute  in 
Philadelphia.  He  finds  in  these  records  an  increment  of  a  little  more 
than  200  in  the  first  year,  but  the  drop  comes  during  the  second  year 
and  is  startling  in  its  enormous  extent  and  is  contrasted  with  the  later 
less  decline.  The  phenomena  may  well  arouse  our  attention  and  con- 
vince us  that  we  are  approaching  a  most  important  scientific  question, 
the  question  of  why  the  drop  comes  in  this  way.  In  the  case  of  girls, 
as  the  next  of  our  slides  will  show,  we  can  prove  the  same  phenomena 
with  slightly  different  values.  Girls,  like  the  females  of  other  species, 
grow  a  little  less  forcibly,  so  to  speak,  than  boys.     They  do  not  quite 


1 00% 


20% 
"0% 


\_^_^                                  -T~-T-^-^ 

^1  r              r  1  \  \ — r^i 

9      10     M      12     13     14     15      16      17     18      19    20    21     22    23 
Fig.  32. 


attain  a  200  per  cent,  value  for  the  first  year,  but  they  too  drop  in  a 
similar  manner  to  the  boys  to  about  30  per  cent.,  and  away  down 
towards  10  per  cent,  in  the  third  year.  Then  comes  the  long  slow 
gradual  decline  up  to  the  period  of  twenty-three.  Professor  Donaldson, 
as  our  next  slide  will  demonstrate  to  us,  has  prepared  curves  from  the 
English  figures  for  girls  also.     They  come  up  nearer  to  the  200  per 


208 


POPULAR    SCIENCE   MONTHLY 


cent,  than  in  Miihlmann's  table,  but  drop  well  below  30  per  cent,  in  the 
second  year,  and  down  to  20  per  cent,  in  the  fourth.  Then  occurs  the 
slight  increase  of  growth  in  the  period  of  twelve,  thirteen,  fourteen 
years,  and  next  the  final  stage  of  decline.  In  the  four  cases  the 
human  rate  curve  is  similar.  The  great  fall  takes  place  at  the 
beginning,  the  slow  fall  towards  the  end.  Professor  Thoma  has 
thought  he  could  get  somewhat  more  accurate  results  by  putting  boys 


100% 


30% 


— 

N 

\ 

r\       ^~-~^             — -T^^ 

r^-- 

rr    ^~' 

^■^--^^-^^ 

10      II        12       13      14      15      16       17      la 

Fig.  33. 


19     20     21      22     23 


and  girls  together,  and  he  has  made  a  calculation,  as  shown  now  upon 
the  screen,  of  a  curve  in  which  the  two  sexes  are  combined.  His  figures 
again  differ  somewhat  from  those  we  have  considered,  but  you  meet  in 
this  curve  also  the  same  general  phenomena.  There  is  an  enormous 
percentage  of  growth  during  the  first  year;  an  enormous  drop  during 
the  second;  then  the  slow  decline;  the  moderate  fluctuation  upward; 
and  then  the  last  slow  disappearance  of  growth.  In  every  instance, 
therefore,  we  have  an  absolute  demonstration,  it  seems  to  me,  of  the 
strange  phenomenon.  Paradoxical  it  will  sound,  whenever  it  is  first 
stated  to  any  one,  that  the  period  of  youth  is  the  period  of  most  rapid 
decline;  that  the  period  of  old  age  is  that  in  which  decline  is  slowest. 
We  shall  learn  in  the  next  lecture  that  this  double  phenomenon  fur- 
nishes us  a  clue  to  further  investigations,  and  leads  to  certain  new 
inquiries,  which  enable  us  to  gain  some  further  insight  into  the  essential 
nature  of  the  phenomena  of  age. 


AGE,    GROWTH   AND   DEATH 


209 


•00% 


30% 


This  completes  the  series  of  curves  which  I  had  prepared  to  present 
to  you  to  show  the  rate  of  growth  in  animals  from  their  birth  only,  but 
of  course  there  has  been  also  a  growth  of  the  animals  which  preceded 
their  birth,  and  that  now  must  briefly  be  considered. 

The  mere  inspection  of  developing  embryos  of  known  ages  gives  us 
some  idea  of  the  rate  of  growth.  With  the  aid  of  the  lantern  I  will 
ask  you  to  look  with  me  at  some  pictures  of  the  developing  chick  and 
developing  rabbit.     Let  us  begin  with  the  chick.^ 

*  During  the  lectures  a  series  of  lantern  slides  were  projected  upon  the 
screen,  made  from  photographs  of  mounted  specimens  of  chicken  embryos,  which 
showed  very  clearly  the  progress  of  development  in  the  chick  during  the  very 
early  stages.  The  first  figure  illustrated  a  chick  of  18  hours'  incubation.  The 
embryo  had  been  skimmed  off  from  the  surface  of  the  ^gg,  hardened,  colored 
artificially  and  mounted  in  the  manner  of  the  ordinary  microscopical  prepara- 
tion in  Canada  balsam.  At  this  age  the  naked  eye  can  just  distinguish  a  line, 
which  indicates  the  position  of  the  axis  of  the  embryo.  The  unaided  eye  can 
recognize  nothing  more.  In  the  second  picture  the  head  and  neck  of  the  embryo 
were  easily  distinguishable,  and  a  few  of  the  earliest  primitive  segments.  The 
third  slide  showed  a  stage  of  a  day  and  a  half.  The  spinal  cord  and  brain  were 
distinctly  differentiated,  and  numerous  so-called  "  blood  islands  "  scattered  about 

VOL.  LXXI. — 14. 


2IO  POPULAR    SCIENCE   MONTHLY 

We  have  first  an  embryo  of  twenty  hours  of  incubation;  following 
it  one  of  one  day.  You  can  observe  just  a  little  line  of  structure  indi- 
cated and  showing  where  the  longitudinal  axis  is  to  be  situated.  By 
the  second  day  the  chick  has  distinctly  a  head  and  a  little  heart,  and 
those  who  are  expert  can  differentiate  with  a  microscope  the  axis  of  the 
body,  the  beginning  of  the  formation  of  the  intestine  and  of  the  mus- 
cles. At  the  end  of  the  first  day  there  was  little  more  than  a  mere 
gathering  of  cells,  but  during  the  twenty-four  hours  of  the  second  day 
the  gathering  has  changed  from  a  mere  streak  upon  the  surface  of  the 
yolk  to  a  well-formed  individual,  with  recognizable  parts  and  several 
times  the  volume  it  had  when  one  day  old.  The  next  figure  illustrates 
the  alteration  which  occurs  during,  approximately,  the  third  day.  It 
is  obvious  that  the  embryo  has  again  made  an  enormous  increase  in 
volume.  The  eye  has  developed,  the  heart  has  become  large,  the  tail 
is  projecting,  the  dorsal  curve  of  the  future  neck  is  distinguishable. 
We  pass  next  to  the  fourth  day.  Is  it  not  a  strange  looking  beast,  with 
its  wing  here  and  leg  there,  a  little  tail  at  this  point ;  an  enormous  eye, 
almost  monstrous  in  proportion;  and,  finally,  here  a  bit  of  the  brain. 
After  five  days  we  have  a  chick  the  brain  of  which  is  swelling,  causing 
the  head  to  be  of  so  queer  a  shape  that,  with  the  eye,  which  seems  out 
of  all  proportion  to  the  rest  of  the  body,  it  imparts  an  uncanny  look 
to  the  embryo.  The  wing  is  shaping  itself  somewhat,  and  the  ends  of 
the  leg,  we  can  see,  will,  by  expansion,  form  a  foot.  Finally,  the  chick 
after  seven  and  after  eight  days  is  figured.  In  the  short  interval  of 
only  six  days  the  chick  grows  from  the  size  represented  by  Fig.  2  to 
that  shown  in  the  last  figure  upon  the  plate.  It  is  an  enormous  in- 
crease. Suppose  a  chick  after  it  was  born  were  to  grow  at  such  a  rate 
as  that!  The  eight-day  embryo  is  thirty  or  forty  times  as  big  as  it 
was  eight  days  before.  It  would  seem  marvelous  to  us  if  a  chick 
after  it  was  hatched  should  become  in  eight  days  thirty  times  as  large 
and  heavy  as  when  it  first  came  out  from  the  Qgg.  It  is  perhaps  advis- 
able to  let  you  follow  the  growth  of  the  chick  a  little  farther,  and 
accordingly  I  present  another  picture  which  shows  an  embryo  of 
about  ten  days.  The  little  marks  upon  the  surface  of  these  embryos 
indicate  the  commencing  formation  of  the  feathers.  A  comparison  of 
the  series  of  figures  proves  that  the  development  is  taking  place  with 
marvelous  speed.  We  need  only  to  look  at  these  stages,  comparing  them 
with  one  another,  to  realize  that  the  progress  of  the  embryo  in  size  and 
development  occurs  with  a  rapidity  which  is  never  to  be  found  in  later 
stages. 

The  history  of  embryonic  rabbits  declares  with  equal  emphasis  that 
the  earliest  development  is  extremely  rapid.     I  wish  now  to  show  you 

The  final  slide  of  the  series  showed  a  chick  of  three  and  one  half  days.  It  has 
not  seemed  necessary  to  reproduce  these  figures  with  the  present  text,  as  they 
merely  duplicate,  on  a  larger  scale  and  with  more  detail,  the  pictures  which  have 
been  included. 


AGE,    GROWTH   AND    DEATH 


I 


Fig.  35.  Ten  Stages  of  the  Developing  Chick,  after  Franz  Keibel.  All  the  figures  are 
magnified  four  diameters.  In  No.  1  only  the  parts  indicated  in  the  vertical  axis  of  the  figure 
correspond  to  embryonic  structures  proper. 

No.  1.  Incubated  20  hrs.  No.    6.  Incubated  3  days,  16  hrs. 

No.  2.  "  24  hrs.  No.    7.  "  4  days,   8  hrs. 

No.  3.  "  2  days.  No.   8.  "  5  days,   1  hr. 

No.  4.  "  2  days,  19  hrs.  No.    9.  "  7  days,   4  hrs. 

No.  5.  "  2  days,  22  hrs.  No.  10.  ."  8  days,    1  hr. 


a  series  of  pictures  to  illustrate  in  the  same  manner  the  progressive 
development  of  the  rabbit.  Numbers  one  to  five  of  the  figures  upon 
the  screen  represent  what  is  known  as  the  germinal  area,  in  the  center 


212 


POPULAR   SCIENCE  MONTHLY 


of  which  the  actual  embryo  is  gradually  formed.  In  No.  1  merely  the 
axis  is  indicated,  in  front  of  and  alongside  of  which  the  parts  of  the 
embryo  are  to  arise,  as  is  suggested  by  Nos.  2,  3,  4,  5.  These  stages 
cover  the  seventh  and  eighth  days.  Nos.  6  to  14  figure  actual  embryos, 
No.  6  of  nine  and  a  half.  No.  14  of  fifteen  days.  No.  6  is  singularly 
twisted  into  a  spiral  form,  the  reason  for  which  is  still  undiscovered. 
No.  9  shows  the  condition  at  eleven  days — notice  the  limbs,  a  leg  in 
front  and  a  leg  behind,  each  only  a  small  mound  as  yet  upon  the  sur- 


Fig.  36.    A  Chick  Removed  from  an  Egg,  which  had  been  Incubated  10  Days  and 
2  Hours.    Magnified  four  diameters.    After  Keibel. 

face  of  the  body ;  the  distinct  eye,  the  protuberance  caused  by  the  heart. 
Nos.  11  and  12  show  the  embryonic  shape  at  twelve  and  a  half  and  at 
thirteen  days — there  has  been  a  great  increase  of  size  with  accompany- 
ing modifications  of  form.  The  next  pair,  Nos.  13  and  14,  present  us 
embryos  of  fourteen  and  fifteen  days,  respectively,  and  you  see  that 
the  growth  is  very  marked  indeed,  and  the  change  of  form  obvious; 
the  creature  is  now  changing  from  the  embryonic  type  into  something 
resembling  a  rabbit.  Other  pictures  could  readily  be  added,  but,  though 
two  weeks  must  still  elapse  before  the  animal  will  be  ready  to  enter 
the  world,  it  is  not  necessary  for  my  present  purpose  to  include  this 
period  in  our  survey.  We  need  only  contemplate,  it  seems  to  me,  the 
series  of  drawings  in  Fig.  37  to  realize  that  the  early  embryonic  growth 


AGE,    GROWTH   AND   DEATH 


213 


of  the  rabbit,  like  the  embryonic  growth  of  the  chick,  proceeds  with  a 
speed  which  is  never  paralleled  by  the  growth  during  later  stages. 


Fig.  37.  Fourteen  Stages  of  the  Developing  Rabbit,  after  Minot's  and  Taylor's  "  Nor- 
mal Plates."  All  the  figures  are  magnified  four  diameters.  Nos.  2  to  5  are  irregiilar  as  to  age, 
but  show  successive  stages  of  development.  The  early  development  is  extremely  variable  and 
the  observations  do  not  yet  suffice  to  determine  the  average  typical  condition  for  each  day 
under  nine. 

No.  1.  Embryo  of  7^  days.  No.   8.  Embryo  of  103^  days. 


No.  2. 

814     " 

No.  3. 

8^     " 

No.  4. 

'            8         " 

No.  5. 

S%     " 

No.  6. 

9K    " 

No.  7. 

10 

No.    9. 

11 

No.  10. 

31^ 

No.  11. 

12}^ 

No.  12. 

13 

No.  13. 

14 

No.  14. 

15 

214  POPULAR    SCIENCE   MONTHLY 

Now  I  had  a  considerable  number  of  rabbit  embryos  preserved  in 
alcohol,  and  though  it  was  not  very  accurate  to  weigh  them  as  alcoholic 
specimens,  in  order  to  determine  their  true  weight,  yet  I  resolved  to  do 
so  as  it  was  the  best  means  at  my  disposal  at  the  time.  The  result  of 
that  weighing  was  very  interesting  to  me,  because  it  showed  that  in 
the  period  of  nine  to  fifteen  days  the  rabbit  had,  on  an  average,  added 
704:  per  cent,  to  their  weight  daily;  but  in  the  period  of  from  fifteen 
to  twenty  days,  the  addition  is  very  much  less  than  this,  only  212  per 
cent.  But  these  rabbits  at  ten  days  have  already  had  a  considerable 
period  of.  development  behind  them,  and  as  we  have  discovered  that  the 
younger  the  animal  the  more  rapid  its  growth,  we  are  safe,  it  seems 
to  me — since  we  have  learned  that  from  the  tenth  to  the  fifteenth  day 
there  is  a  daily  increase  of  over  700  per  cent. — in  assuming  that  in  yet 
younger  rabbits  an  increase  of  a  thousand  per  cent,  per  day  actually 
occurs.  That  is  not  so  extraordinary  an  assumption,  for  bacteria  are 
known  to  divide  every  half  hour,  and  if  the  little  bacterium  divides  and 
grows  up  to  full  size  in  half  an  hour,  and  then  divides  again,  it  means 
that  within  a  half  hour  one  bacterium  has  become  two,  and  has  in- 
creased, obviously,  100  per  cent.;  and  if  those  two  again  divide  as  before, 
we  should  have  four  bacteria  at  the  end  of  an  hour — an  increase  of  400 
per  cent.,  and  at  the  end  of  another  half  hour,  of  800  per  cent.,  and 
so  on  ever  in  geometrical  progression.  We  learn,  then,  that  bacteria 
may  in  a  few  hours  add  1,000  per  cent,  to  their  original  weight,  and 
it  is  not  by  any  means  an  exorbitant  demand  upon  our  credulity  to 
accept  the  conclusion  that  in  their  early  stages,  rabbits  and  other  mam- 
mals and  birds  are  capable  of  growing  at  least  1,000  per  cent,  a  day. 
If  this  be  true,  and  it  doubtless  is  true,  we  can  adopt  it  as  a  convenient 
basis  for  comparison.  As  we  learned  from  the  rate  curves,  which  were 
projected  upon  the  screen  earlier  during  the  hour,  the  male  rabbit  gains 
in  one  day  immediately  after  birth  nearly  eighteen  per  cent. — seven- 
teen and  four  tenths  per  cent. — and  the  female  rabbit  gains  nearly 
seventeen  per  cent.  Now  we  can  estimate  the  loss  very  simply  by 
deducting  this  rate,  which  is  the  capacity  of  the  animal  to  grow  per- 
sisting at  birth,  from  its  original  capacity,  which,  we  assume  to  have 
been  1,000  per  cent,  per  day.  And  if  we  do  that  the  result  is  obvious. 
Over  98  per  cent,  of  the  original  growth  power  of  the  rabbit  or  of  the 
chick  has  been  lost  at  the  time  of  birth  or  hatching,  respectively,  and 
the  same  thing  is  equally  true  of  man.  We  start  out  at  birth  certainly 
with  less  than  two  per  cent,  of  the  original  growth  power  with  which 
we  were  endowed.  Over  98  per  cent,  of  the  loss  is  accomplished  before 
birth — less  than  two  per  cent,  after  birth.  That,  I  think  is  a  rather 
unexpected  conclusion,  certainly  not  one  which,  until  I  began  to  study 
the  subject  more  carefully,  I  in  the  least  expected;  and  even  now  when 
I  have  become  more  familiar  with  it,  it  still  fills  me  with  astonishment, 
it  is  so  different  from  the  conception  of  the  process  of  development  as 
we  commonly  hold  it,  from  our  conclusions  based  on  our  acquaintance 


AGE,    GROWTH   AND    DEATH 


215 


300% 


with  the  growth  and  progress  of  the  individuals  about  us.  We  over- 
look the  fact  that  the  progress  which  each  individual  makes  is  the  result 
of  accumulation.  It  is  as  if  money  was  put  into  the  savings-bank;  it 
grows  and  becomes  larger,  but  the  rate  of  interest  does  not  alter.  So 
too  with  us;  we  see  there  is  an  accumulation  of  this  wealth  of  organi- 
zation which  gives  us  our  mature  power.  But  as  that  accumulation 
goes  on,  our  body  seems  to  become, 
as  it  were,  tired.  We  may  com- 
pare it  to  a  man  building  a  wall.  eoo^ 
He  begins  at  first  with  great  en- 
ergy, full  of  vigor;  the  wall  goes 
up  rapidly;  and  as  the  labor  con- 
tinues fatigue  comes  into  play. 
Moreover,  the  wall  grows  higher, 
and  it  takes  more  effort  and  time 
to  carry  the  material  up  to  the 
top  of  the  wall,  and  to  continue  to 
raise  its  height,  and  so,  as  the  wall  400^ 
grows  higher  and  higher,  it  grows 
more  slowly  and  ever  more  slowly, 
because  the  obstacles  to  be  over- 
come have  increased  with  the  very 
height  of  the  wall  itself.  So  it 
seems  with  the  increase  of  the  or- 
ganism; with  the  increase  of  our 
development,  the  obstacles  to  our 
growth  increase.  How  that  is  I 
shall  hope  to  explain  to  you  a  little 
more  clearly  in  the  next  lecture. 

We  have  one  more  slide,  which 
r  would  like  to  show  you.  It 
indicates  the  rate  of  growth  in 
man  before  birth  as  far  as  it 
can  be  indicated  without  better 
knowledge.  The  time  intervals 
in  the  diagram  correspond  to 
tbe  so-called  lunar  months — the 
ten  lunar  months  of  prenatal 
life.  Of  our  early  development  we  know  very  little  so  far  as  statistics 
are  concerned,  but  from  the  third  month  onward  we  have  some  records. 
It  is  found  that  from  the  third  to  the  fourth  month  the  increase  is  600 
per  cent.  Just  contrast  that  with  200  per  cent,  added  in  one  year 
after  birth;  600  per  cent,  in  one  month  against  200  per  cent,  in  one 
year.  From  the  fourth  to  the  fifth  month  it  is  scarcely  over  200  per 
cent.  It  then  becomes  only  a  little  more  than  100.  In  the  seventh 
month,  less  than  100;  and  finally  in  the  ninth  and  tenth  months,  it 


Fig.  3S. 


2i6  POPULAR    SCIENCE   MONTHLY 

becomes  very  small  indeed,  less  than  20,  so  that  during  the  prenatal 
life  of  man,  as  we  have  seen  in  the  prenatal  life  of  the  rabbit  and  of 
the  chick,  the  decline  in  the  power  of  growth  is  going  on  steadily  all 
the  time. 

I  shall  use  the  few  remaining  moments  to  report  to  you  yet  another 
bit  of  evidence  of  the  originally  enormous  power  of  growth.  It  has 
been  estimated  that  the  germ  of  the  mammal,  with  which  the  develop- 
ment commences,  has  a  weight  of  0.6  milligram;  another  estimate 
which  I  have  found  is  of  0.3  milligram.^  Perhaps  I  can  give  you  some 
idea  of  what  this  value  means  by  telling  you  that  if  the  weight  of  the 
original  germ  of  a  mammal  is  assumed  to  be  0.6  milligram,  we  could, 
according  to  the  laws  of  the  United  States,  send  50,000  such  germs 
by  letter  postage  for  two  cents.  It  would  take  50,000  germs  to  make 
the  weight  of  one  letter.  That  perhaps  will  give  you  some  impression 
of  the  extreme  minuteness  of  the  primitive  germ.  In  the  human 
species  at  the  end  of  even  a  single  month  it  is  no  longer  merely  a  germ, 
but  a  young  human  being,  very  immature,  of  course,  in  its  development, 
but  already  very  much  larger.  I  doubt — even  after  all  that  I  have  said 
this  evening  about  the  startling  figures  of  growth  for  the  earlier 
stages, — I  doubt  if  you  are  prepared  for  the  fact  that  the  growth  of  the 
germ  up  to  the  time  of  birth  represents  an  increase  of  over  five 
million  per  cent.  How  much  over  five  million  per  cent,  we  can  not 
calculate  accurately,  because  we  do  not  know  accurately  the  weight  of 
the  original  germ,  but  an  increase  of  five  million  per  cent,  is  not  above 
the  true  value.  Contrast  that  with  anything  which  occurs  in  the  later 
periods.  What  a  vast  change  has  happened!  What  an  immense  loss 
has  taken  place !  The  rate  of  this  loss  is  evidently  diminishing.  The 
less  occurs  with  great  rapidity  in  the  young — less  rapidity  the  older  we 
become.  I  attempted  to  convince  you  in  the  first  and  second  lectures 
that  that  which  we  called  the  condition  of  old  age,  is  merely  the  culmi- 
nation of  changes  which  have  been  going  on  from  the  first  stage  of 
the  germ  up  to  the  adult,  the  old  man  or  woman.  All  through  the  life 
these  changes  continue.  The  result  is  senility.  But  if,  as  the  phe- 
nomena of  growth  indicate  to  us  so  clearly,  it  be  true  that  the  decline 
is  most  rapid  at  first,  then  we  must  expect  from  the  study  of  the  very 
young  stages  to  find  a  more  favorable  occasion  for  analysis  of  the 
factors  which  bring  about  the  loss  in  the  power  of  growth  and  change 
as  the  final  result  of  which  we  encounter  the  senile  organism,  JSTot 
from  the  study  of  the  old,  therefore,  but  from  the  study  of  the  very 
young,  of  the  young  embryo,  and  of  the  germ,  are  we  to  expect  insight 
into  the  complicated  questions  which  we  have  begun  to  consider  together. 
I  shall  hope  in  the  next  lecture  to  prove  to  you  that  the  supposition 
which  has  guided  my  own  observations  is  correct,  and  to  be  able  to 
show  you  that  we  do  actually,  from  the  study  of  the  developing  embryo, 
glean  some  revelations  of  the  cause  of  old  age. 

*  These  estimates  refer  to  the  placental  mammals  only. 


AGE,    GROWTH   AND    DEATH  359 


THE  PKOBLEM  OF  AGE,  GEOWTH  AXD  DEATH 

By  CHARLES  SEDGWICK  MINOT,  LL.D.,  D.Sc. 

JAMES  STILLMAN   PROFESSOR  OF  COMPARATIVE  ANATOMY  IN   THF,  HARVARD 
MEDICAL  SCHOOL 

IV.     Differentiation  and  Eejuvenation 

Ladies  and  Gentlemen:  In  order  to  present  the  subject  of  this 
evening,  I  will  take  a  few  brief  moments  at  the  beginning  to  review 
the  results  reached  in  the  previous  lecture.  In  the  last  lecture  I 
spoke  of  the  phenomena  of  growth,  and  endeavored  then  to  make 
clear  to  you  what  I  consider  the  fundamental  conception  of  this  study 
— that  the  decline  in  the  growth  power  is  extremely  rapid  at  first  and 
slow  afterwards.  This  change  in  the  rate  of  growth  is  of  course  due  to 
things  in  the  animal  body  itself.  It  is  a  logical  conclusion  for  us  to 
draw  that  if  we  are  to  study  out  the  cause  of  the  loss  of  growth  power, 
we  should  do  it  rather  at  that  period  of  development  when  the  change 
in  the  rate  of  growth  is  most  rapid,  for  then  we  should  expect  those 
modifications  to  exhibit  themselves  most  clearly  because  the  magnitude 
of  cause  is  likely  to  be  proportionate  to  the  magnitude  of  result,  and 
when  the  decline  is  most  rapid,  then  we  must  expect  to  find  the  altera- 
tions which  cause  that  decline  in  the  organism  to  show  themselves 
most  conspicuously.  You  will  remember,  further,  that  we  spoke  of 
growing  old  as  being  a  much  more  complicated  question  than  one  of 
growth  alone,  and  that  there  occur,  as  the  years  advance,  changes  in 
the  structure  of  the  body.  It  is  convenient  to  use  one  collective  term 
for  all  these  phenomena  of  becoming  old,  and  that  term,  established 
by  long  usage,  is  senescence,  the  becoming  old.  Wliat,  therefore,  w^e 
have  to  search  for  at  present  is  a  cause,  a  proximate  cause  at  least, 
of  senescence.  In  order  to  make  the  view  I  am  to  bring  forward  this 
evening  quite  clear  to  you,  I  must  first  of  all  take  advantage  of  your 
kindness  and  recapitulate  briefly  what  I  said  in  regard  to  cells,  for 
you  will  remember  that  the  cell  is  the  foundation  and  unit  of  organic 
structure.  With  your  permission  I  should  like  to  recall  more  exactly 
to  your  minds  what  I  said  of  the  cells  by  having  thrown  upon  the 
screen  the  slide  which  we  saw  before  and  which  we  used  as  an  illustra- 
tion of  the  cell.  Here  is  the  picture.  Above  we  see  the  typical  cell 
from  the  oral  epithelium  of  the  salamander,  and  you  remember  in  the 
center  this  more  conspicuous  body  wdth  a  granular  and  reticulated 
structure  which  we  called  the  nucleus,  and  surrounding  it  is  this  mass 
which  we  called  the  body  of  the  cell,  or  the  protoplasm.     Here  is  an- 


:6o 


POPULAR    SCIENCE   MONTHLY 


other  condition  of  a  cell  of  the  skin  of  the  salamander  in  which  the 
nucleus  j^resents  a  slightl}'  different  appearance.  Here  also  we  have 
quite  a  hody  of  protoplasm  about  the  nucleus.  Every  cell  consists  of 
these  two  essential  and  fundamental  parts,  the  nucleus  and  the  proto- 
jDlasm.     Now  the  conclusion  to  which  I  shall  gradually  bring  you  by 


J  -^■■•'-    '^ 


-     ^ 


.-'/' 


rh 


'-^^ 


/O 


Fig.  39.    Cells  from  the  Mouth  (Oral)  Epithelium  of  the  Salamander. 

the  facts  to  be  laid  before  you  this  evening  is  that  the  increase  of  the 
protoplasm  is  the  thing  which  is  to  be  regarded  as  the  explanation  of 
senescence.  Though  protoplasm  is  the  physical  basis  of  life,  though 
It  IS  the  actual  living  substance  of  the  body,  its  undue  increase  bevond 
the  growth  of  the  nucleus  changes  the  proportions  of  the  two,  and'that 
change  of  proportion  seems  to  cause  an  alteration  in  the  conditions  of 


AGE,    GROWTH   AND    DEATH  361 

the  living  cell  itself,  and  that  alteration  I  inteq^ret,  as  I  shall  explain 
more  accurately  later,  as  the  cause  of  senescence,  as  the  fundamental 
cause  of  old  age.  This  slide  also  shows  to  us  the  early  development  of 
the  cells  through  those  phases  which  result  in  the  multiplication  of 
them.  The  nucleus  changes  in  appearance  and  becomes  a  very  differ- 
ent-looking structure.  These  changes  I  need  not  now  go  through 
again.  Suffice  it  to  say  that  after  the  complicated  alterations  have 
completed  their  cycle,  we  get  in  the  place  of  a  single  cell,  two,  and 


^pi^sfeK, 


^^^^m^^m^^mB^m 


^rm:m> 


•-^  - '^^^'ii  'W'"'-^  -'^'-  ^"^ 


t-s^" 

€ 

^^kS^i^'^'^'"^ 


13 


Fig.  40.    Three  Sections  through  a  Rabbit  Embryo  of  Seven  and  One  Half  Days. 

each  has  its  own  nucleus,  and  each  its  own  protoplasm.  Xotice  here 
that  the  two  cells  which  finally  result  are  smaller  than  the  original 
cells  from  which  they  sprang.  These  are  by  no  means  imaginary 
pictures,  but  accurate  microscopic  drawings  from  real  cells  of  the 
salamander  skin.  The  two  cells  which  are  thus  produced  from  one 
parent  cell  are  characterized  by  their  smaller  size,  and  this  smaller  size 
applies  not  only  to  the  cell  as  a  whole,  but  likewise  to  its  nucleus. 
After  having  been  thus  reduced  in  size,  the  nuclei  and  the  cells  will 
both  expand,  and  soon  the  daughter  cells  will  return  to  the  mother 
dimension  and  be  as  large  as  the  parent  cell  from  the  division  of  which 


362 


POPULAR    SCIENCE   MONTHLY 


Fig.  41.  ^OTce6a  ro/t,  HIGHLY  magnified.  Drawn 
from  a  cover-glass  preparation  Irom  a  twenty-four 
hour  culture. 


they  arose.  There  is  thus, 
we  learn,  the  constant 
fluctuation  in  the  size  of 
cells,  a  fluctuation  in  their 
dimensions  accompanying 
the  process  of  cell  division. 
Presently  we  shall  have 
more  to  say  in  regard  to 
this  matter  of  the  change 
in  the  cell  in  size.  The 
next  picture  (Fig.  40) 
which  I  want  to  recall  to 
you  is  one  which  we  also 
had  in  an  earlier  lecture. 


These  represent  slices  through 
a  very  young  rabbit  before 
any  of  the  organs  of  the 
rabbit  have  begun  to  develop. 
We  can  see  here  clearly  the 
nuclei,  as  I  pointed  out  to 
you  before,  nearly  uniform 
in  structure,  and  you  notice 
that  the  protoplasm  around 
each  nucleus  is  quite  small 
in  amount.  If  you  will  re- 
call the  previous  picture  of 
the  skin  of  the  salamander, 
upon   the   screen   a   moment 


\  . 


Fig.  43.  rrypanosoma  Lewisi,  from 
blood  with  two  blood  corpuscles  alongside 
the  same  scale. 


KiG.  42.  Tekttan  Malarial  Parasite.  Two 
huiDfin  blood  corpuscles  alongside  and  drawn  on 
the  same  scale. 

ago,  you  will  realize  imme- 
diately, in  comparing  the 
two,  that  in  these  young  cells 
the  proportion  of  the  proto- 
plasm to  the  nucleus  is  very 
small.  That  is  again  one  of 
the  fundamental  facts  to 
which  we  shall  recur  in  a 
moment.  I  wanted  to  show 
you  this  picture  in  order  to 
revive  in  your  minds  the  con- 
ception which  I  endeavored 
to  give  you  before  of  the 
undifferentiated  tissue,  where 
the  cells  have  nuclei  pretty 


the  rat's 
drawn  on 


AGE,    GROWTH   AND   DEATH  363 

uniform  in  appearance  and  in  size,  each  with  its  little  mass  of  proto- 
plasm about  it,  and  this  protoplasm  appearing  in  all  the  cells  under 
microscopic  examination  very  much  the  same.  We  can  not  in  this 
stage  of  development  say  of  a  given  cell  that  it  represents  any  special 
structure,  by  which,  if  we  saw  it  isolated  under  the  microscope,  we 
could  determine  from  what  part  of  the  young  embryonic  body  it  was 
derived.  When  we  see  a  cell  from  the  adult  we  can  determine  its 
origin  with  certainty  by  its  microscopic  appearance  alone.  As  devel- 
opment progresses,  the  simple  condition  of  the  cells  is  gradually  oblit- 
erated, but  we  find  another  condition  arising  which  we  call  the  differ- 
entiated one.  Differentiation  is  a  process  which  goes  on  in  the  body 
as  a  whole,  but  of  course  it  is  also  a  function  of  each  individual  cell. 
We  can  see  something  of  the  process  of  differentiation  if  we  study  the 
unicellular  organisms,  those  creatures,  each  of  which  is  complete  in 
itself,  although  it  consists  of  but  a  single  cell,  not  of  countless  millions 
of  cells  as  we  do.  The  picture  (Fig.  41)  which  I  have  chosen  to  throw 
upon  the  screen  is  one  which  I  think  might  have  an  additional  interest 
to  you,  for  it  is  a  photograph  from  the  living  cell  known  as  the  para- 
site producing  dysentery.  Its  scientific  name  is  amoeba  coli.  It  is  a 
photograph  from  life.  Here  vaguely  in  the  center,  marked  with  a 
finer  granulation,  and  some  of  the  darker  spots  in  it,  we  can  distin- 
guish the  nucleus :  here  is  the  outline  of  the  protoplasm  of  this  cell, 
and  in  it  are  included  some  particles  of  food  which  this  protoplasmic 
body  has  absorbed  for  purposes  of  digestion.  This  is  a  unicellular 
parasitic  organism  with  scarcely  any  differentiation  of  its  structure. 
The  next  of  the  slides  shows  us  again  another  of  these  parasitic  simple 
organisms.  The  figure  here  to  the  right  of  the  field  of  view  is  the 
one  which  should  especially  attract  your  attention.  The  other  two 
bodies  near  it  are  blood  corpuscles,  human  blood  corpuscles.  The 
organism  in  this  case  is  the  one  which  causes  malarial  fever,  and  it  is 
in  a  particular  stage  of  its  development;  that  which  we  distinguish 
as  the  tertian  malarial  parasite  is  the  one  here  represented.  You  can 
see  in  this  case  also  the  outline  of  the  nucleus,  surrounded  by  the 
protoplasm — the  whole  thing  only  a  little  bigger  than  a  single  human 
blood  corpuscle.  Here  also  we  note  the  absence  of  differentiation. 
Another  stage  of  this  same  tertian  malarial  parasite  is  shown  next. 
This  I  have  projected  upon  the  screen  because  it  illustrates  more  clearly 
than  the  other  the  nucleus  and  the  small  amount  of  protoplasm  about 
the  nucleus.  The  malarial  organism  is  one  of  great  vitality,  capable 
of  enormously  rapid  multiplication,  and  it  undoubtedly  owes  that 
faculty  to  its  constitution,  to  the  relation  between  the  nucleus  and  the 
protoplasm.  I  will  now  show  you  another  picture  of  parasites — one 
form  of  which,  in  a  related  species,  occurs  in  man.  This  particular 
form  is  one  which  occurs  in  the  rat  and  is  called  the  Trypanosoma. 
You  can  see  that  the  body,  instead  of  being  a  small  and  sirnj)le  struc- 


364 


POPULAR    SCIENCE   MONTHLY 


ture^  has  elongated,  acquired  a  peculiar  form,  and  here  in  the  interior 
are  lighter  and  darker  spots.  These  do  not  show  very  clearly  in  the 
picture,  because  it  is  from  a  photograph  of  a  living  specimen  under 
the  microscope.  The  lighter  and  darker  spots  correspond  to  the  de- 
tails in  the  structure  of  the  organism.  Here  is  the  tail  of  the  organism, 
twisted,  as  you  see,  and  in  life  capable  of  being  bent.  The  movement 
of  the  animals  in  the  natural  fluid  in  which  they  are  suspended  is 
quite  active.  Alongside  are  some  blood  corpuscles,  the  figure,  as  you 
see,  is  magnified  about  the  same  as  the  one  of  the  malarial  parasite 
which  I  showed  you  a  few  moments  ago.     The  next  slide  exhibits  an 


B 


i 


Fig.  ,44.    Stenior  coeruleus    ^,  cut  into  three  pieces  ;  i?,  regeneration  of  the  first  piece; 
C,  of  the  middle  piece  ;  D,  of  the  posterior  piece.    After  Gruber." 

organism  which  swims  free  in  the  water,  and  is  pretty  M^ell  shown  in 
this  figure.  It  is  called  the  Stenior.  Here  the  chain  of  beads  repre- 
sents the  nucleus.  Upon  the  surface  of  the  body  there  are  fine  lines 
indicating  superficial  structure.  At  this  point  there  occurs  what  we 
call  the  mouth.  Over  the  rest  of  this  minute  organism  there  is  a 
thin  cuticle,  but  at  the  mouth  the  cuticle  is  absent,  and  the  protoplasm 
is  naked  or  uncovered  so  that  food  can  be  taken  in.  There  are  bands 
of  hairs  showing  coarse  and  stiff  in  the  figure  but  capable  of  movement, 
and  with  the  aid  of  those  vibratile  hairs,  or  cilia,  the  organism  can 
swim  about  in  water.  Here  is  another  internal  structure,  the  vacuole ; 
obviously  in  an  animal  like  this  we  no  longer  have  simple  protoplasm 


AGE,    GROWTH   AND    DEATH  365 

alone,  but  the  protoplasm  in  the  interior  of  the  cell  has  become  in  part 
changed  into  other  things.  Here  then  within  the  territory  of  a  single 
cell  Ave  have  differentiation.  If  now  in  these  unicellular  organisms 
we  study  both  the  protoplasm  and  the  nucleus,  we  learn  that  most  of 
these  modifications  which  are  so  conspicuous  upon  microscopic  observa- 
tion are  due  to  changes  in  the  protoplasm.  It  is  the  protoplasm  which 
acquires  a  new  structure.  In  the  nucleus,  on  the  contrary,  we  find 
perhaps  a  change  of  form,  minor  details  of  arrangement  by  which  one 
sort  of  nucleus,  or  one  stage  of  the  nucleus,  can  be  distinguished  from 
another,  but  always  the  nucleus  consists  of  the  same  fundamental 
constants.  There  is  the  membrane  bounding  it ;  there  is  the  sap  or 
juice  in  the  interior;  the  network  of  living  threads  stretching  across 
it;  and  here  and  there  imbedded  in  and  connected  with  this  network 
are  the  granules  of  special  substance,  which  we  call  chromatin.  These 
four  things  exist  in  the  nuclei  and  are  apparently  always  present,  and 
there  is  usually  not  to  be  seen  in  the  nucleus  anything  of  change  com- 
parable, in  extent  at  least,  with  the  change  which  goes  on  in  the  proto- 
plasm— on  the  other  hand,  the  protoplasm  acquires  items  of  structure 
which  were  totally  absent  from  it  before.  The  nucleus  rearranges  its 
parts  rather  than  changes  them.  This  is  a  very  important  fact,  and 
shows  us,  if  we  confine  our  attention  even  to  these  little  organisms 
only,  that  the  differentiation  of  the  protoplasm  is  quant iiatively  the 
more  important  of  the  two — the  differentiation  of  the  nucleus  the 
less  important. 

We  can  now  turn  from  a  consideration  of  these  lowest  organisms 
to  the  higher  forms,  among  which  we  ourselves  of  course  are  counted, 
in  which  the  body  is  formed  by  a  very  considerable  number  of  cells. 
Again  I  should  like  to  take  advantage  of  your  kindness  and  show  you 
some  of  the  pictures  we  have  already  reviewed,  in  order  to  utilize  the 
features  which  they  show  as  illustrations  of  the  fundamental  principle 
that  the  conspicuous  change  is  in  the  protoplasm.  Here  we  have  nerve 
cells.  In  the  first  two  photographs  are  represented  two  isolated  nerve 
cells,  to  show  their  shape.  They  have  been  colored  by  a  special  process 
so  dark  that  the  nucleus  which  they  contain  in  their  interior  is  hidden 
from  our  view;  it  is  of  course  none  the  less  there.  This  dark  stain- 
ing enables  us  to  trace  out  the  shape  of  these  cells  very  clearly,  and 
you  can  see  that  instead  of  being  round  and  simple  in  form  they  have 
their  elongated  processes  stretching  out  to  a  very  considerable  dis- 
tance; these  processes  serve  to  catch  up  from  remote  places  nervous 
impulses  and  carry  them  into  the  body  of  the  cell,  and  thus  assist  in 
the  work  of  nervous  transmission.  The  elongation  of  these  threads  is, 
as  you  see,  adapted,  like  tlie  elongation  of  a  wire,  to  long-distance 
communication.  Here  are  two  other  figures  which  represent  nerve 
cells  treated  by  a  different  process,  and  again  artificially  colored.  But 
the  color  in  this  case  has  attacked  certain  spots  in  the  protoplasm, 


z(>^ 


POPULAR    SCIENCE   MONTHLY 


consequent!}^  we  see  that  the  protoplasm  around  the  nucleus  in  both 
of  these  tigures  is  no  longer  simple  and  uniform,  but  contains  these 
deposits  of  dark-colored  material.    Here  are  other  nerve  cells;  the  one 


lig.l. 


pM^' 


mi 


'0 


bif 


& 


©  ^^l. 


p 


Fig.  4. 


%i 


Fig. 


Fig./J. 
Fig.  46.    Various  Kinds  of  Human  Nerve  Cells.    After  Sobotta. 

in  the  center  shows  you  the  accumulation  of  pigmented  matter  in  the 
protoplasm;  again  an  index  of  a  change  and  lack  of  the  previous  uni- 


AGE,    GROWTH   AND    DEATH 


367 


formity  replaced  by  diversity  in  the  composition  of  the  various  parts 
of  the  single  cell.  This'  figure  shows  us  more  clearly  the  principle 
of  structure  of  a  nerve  cell,  for  here  we  have  the  central  body  of  the 
cell  composed  of  protoplasm  with  its  nucleus  in  the  middle  and  a 
small  spot  in  the  center  of  the  nucleus,  and  these  long  branching 
processes  running  out  in  all  directions  which  can  take  up  nerve  im- 
pulses from  other  similar  or  dissimilar  cells,  as  the  case  may  be,  and 
carry  them  to  the  central  body.  To  carry  the  message  out  there  is 
typically  but  one  process,  which  is  different  in  appearance  from  the  other 
processes  which  carry  the  impulses  in.  The  latter  are  branching  and 
are  therefore  called  the  tree-like  or  dendritic  processes.  Here  is  a 
single  process  like  a  long  thread  to  carry  the  impulses  away,  and  which 


t*- — 


'l^- 


MM».Mi<iai«irrft*wi  liVi 


Fig.  46.    Part  of  a  Human  Muscle 
Fiber. 


Fig.  47.    Section  from  an  Orbital  Gland. 


is  called  the  axon  of  the  nerve  cell.  In  this  case  the  modification  of 
the  shape  of  the  cell  has  adapted  it  to  the  better  performance  of  its 
functions.  Xotice  also  in  these  cells  the  enormous  increase  in  the 
amount  of  protoplasm  as  compared  with  the  nucleus.  In  the  young 
cell  of  the  rabbit  germ,  of  which  I  showed  you  several  illustrations  a 
few  moments  ago,  we  had  very  little  protoplasm  for  each  nucleus,  but 
here  the  protoplasm  has  many,  many  times  the  volume  of  the  nucleus, 
and  this  is  a  relatively  old  cell. 

Xext  let  us  look  again  at  the  figure  of  the  striated  muscle  fiber, 
which  you  may  recall  from  the  second  lecture,  so  that  it  will  suffice 
if  your  attention  is  again  directed  to  the  oval  nuclei,  and  to  the  lines 
stretching  crosswise  on  the  muscle  giving  it  a  "  striated  "  appearance. 
You  remember,  doubtless,  that  such  fibers  are  the  ones  which  enable 
us  to  make  voluntary  motions.      Originally  each  fiber  was  a  set  of 


368  POPULAR    SCIENCE   MONTHLY 

cells,  and  the  cells  had  some  protoplasm,  but,  gradually,  as  develop- 
ment progressed,  there  appeared  in  them  longitudinal  fibrils  different 
from  the  protoplasm,  and  the  fibrils  also  created  ultimately  the  appear- 
ance of  cross  lines  on  the  fiber.  It  is  the  fibrils  which  perform  the 
Tnuseular  contractions.  It  is  not  the  original  unmodified  protoplasm, 
but  the  modified  or  difEerentiated  muscular  cell  which  is  capable  of 
voluntary  contraction. 

The  next  picture  (Fig.  47)  shows  us  clearly  and  strilvingly  how  much 
the  differentiation  may  vary.  We  have  here  another  type  of  differen- 
tiation. These  are  gland  cells;  we  can  see  here,  as  I  pointed  out  to 
you  before,  the  material  in  the  form  of  granules,  which  is  to  produce 
the  secretion  from  these  gland  cells.  This  is  an  orbital  gland,  and 
here  are  the  cells,  which  are  very  much  smaller  because  they  have 
discharged  their  secretion.  Three  of  the  cells  are  represented  sepa- 
rately. The  first  shows  us  a  cell  full  of  the  material  which  is  to  be 
discharged  and  is  to  form  a  part  of  the  secretion  of  the  saliva.  The 
second  is  a  cell  which  has  partly  lost  its  accumulated  material,  and 
the  third  is  one  which  has  discharged  it  almost  completely,  so  that  it 
has  become  very  much  reduced  in  size.  We  learn  from  such  structures 
as  these  that  the  size  of  cells  may  vary  also  according  to  their  func- 
tional condition.  We  have  here  a  similar  gland.  This  is  sometimes 
called  the  salivary  gland  of  the  intestine,  better  termed  the  pancreas. 
Here  we  can  see  for  each  of  these  cells  a  nucleus  and  a  body  divided 
into  two  parts,  a  darker  portion  around  the  nucleus  and  a  lighter  })art 
with  little  granules  in  it,  which  represents  the  accumulation  of  ma- 
terial which  is  to  form  the  secretion.  Wlien  the  cells  have  discharged 
their  secretion,  they,  like  the  cells  in  the  salivary  gland,  are  found  to 
have  diminished  in  size  and  become  very  much  smaller  indeed  than  they 
were  in  their  earlier  state  when  charged  with  the  zymogen  destined  to 
be  given  out.  In  this  case  also  we  have  an  illustration  of  a  functional 
variation  in  the  size  of  the  cells.  This  ends  the  series  of  pictures 
which  I  wanted  especially  to  show  to  you  as  illustrating  the  changes 
of  the  cells  as  their  differentiation  progresses.  We  can  see  in  the 
bodies  of  the  cells  the  changes  which  have  occurred. 

Here  is  a  picture  which  teaches  us  one  thing  more  about  these 
cells.  ISTotice  the  scattered  nuclei,  each  surrounded  by  protoplasm, 
completing  the  cell.  The  protoplasm  of  each  of  these  cells  is  con- 
nected across  with  the  protoplasm  coming  from  another,  so  that  the 
whole  set  of  cells  forms  an  irregular  protoplasmic  network.  Now  in 
the  spaces  between  these  cells  are  fine  lines.  These  represent  delicate 
structures  which  we  call  connective  tissue  fibrils,  which  have  a  me- 
chanical function.  By  their  tensile  strength,  their  power  to  resist  and 
pull,  they  give  a  certain  supporting  power  to  the  tissues.  Our  picture 
represents  one  of  the  tissues  which  support  and  connect  other  portions 
of  the  body.     JN'ow  the  fibrils  apparently  lie  entirely  disconnected  from 


AGE,    GEO^yTII    AND    DEATH 


369 


the  cells,  but  a  more  careful  study  of  the  history  of  the  connective 
tissue  has  revealed  the  very  interesting  and  instructive  fact  that  the 
fibrils,  now  separate  from  the  cells,  arose  by  a  metamorphosis  of  the 
protoplasm  of  the  cells — that  they  are  first  formed  out  of  some  of  the 
protoplasm  of  these  cells,  then  split  off  from  them,  and  come  to  lie 
in  the  intercellular  regions,  so  that  here  we  have  another  type  of  cell 
differentiation  brought  to  our  notice,  one  in  which  the  product  is 
separated  from  the  parent  body  to  which  it  owes  its  origin.  Xow  you 
will  perceive  immediately,  if  you  recall  the  series  of  pictures  which 
have  just  passed  before  us  on  the  screen,  very  great  differences  in  the 
types  of  differentiation  which  occur  in  the  body,  and  had  we  time  we 
might  find  a  very  much  larger  range  easily  to  be  represented  before  us. 


) 

/ 


-\.-. 


•it:*  its'         f    ■■  "  li'i  '  I  ^  \      Of. 9 


Fig.  48.    E.mbryonic  Syncytium  from  the  Umbilical  Cord  of  Man;  c,c  cells;  i-^,  fibrils. 

In  the  second  lecture  a  picture  was  projected  upon  the  screen, 
which  showed  motor  nerve  cells  of  various  animals.  You  will  recall 
that  I  directed  your  attention  to  the  fact  that  the  largest  animal,  the 
elephant,  has  the  largest  cells,  and  the  smallest  animals,  the  rat,  the 
mouse  and  the  little  bat,  have  the  smallest  ones.  But  let  me  point  out 
to  you  that  the  question  of  the  size  of  cells  is  exceeding  complex,  and 
that  in  studying  it  we  have  to  exercise  a  great  deal  of  caution.  We 
know  that,  with  the  exception  of  the  nerve  cells  and  to  a  minor  degree 
with  the  exception  of  the  muscle  fibers,  the  cells  in  each  animal  are 
more  or  less  uniform  constants  in  size.  The  cells  of  different  organs 
differ  somewhat  from  one  another.  A  single  organ  may  have  in  its 
different  parts  typical  sizes  of  cells,  but  each  of  these  kinds  of  cells 
has  its  definite  dimensions.     When  one  animal  is  larger  than  another, 

VOL.    LXXI. — 24 


370  POPULAR    SCIENCE   MONTHLY 

it  has  more  cells.  Now  it  is  a  very  important  fact  for  us  that  animals 
have  a  more  or  less  constant  size  of  their  cells.  They  do  not  differ 
from  one  another  by  a  difference  in  the  size  of  their  cells;  the  bigness 
of  an  animal  does  not  depend  upon  the  size,  but  upon  the  number,  of 
its  cells.  We  can,  therefore,  in  studying  the  changes  of  size,  to  which 
I  shall  next  direct  your  attention,  omit  altogether  these  details,  and 
speak  of  the  cells  in  a  general  way  safely  as  having  a  certain  uniform 
or  standard  size.  This  will  save  us  a  great  deal  of  time,  for  we  learn, 
as  we  study  cells,  that  their  size  increases  with  the  age  of  the  animal. 
The  animal,  when  it  is  young,  has  cells  with  a  small  amount  of  proto- 
plasm. And  that,  you  will  perceive  from  the  pictures  which  have  been 
thrown  upon  the  screen,  is  an  absolutely  necessary  corollary  of  the 
discovery  that  differentiation  is  mainly  a  function  of  the  protoplasm. 
If  there  is  to  be  a  large  degree  of  differentiation  it  is  necessary  that  the 
quantity  of  protoplasm  in  the  single  cells  should  be  increased,  so  that 
there  may  be  the  raw  material  on  hand  out  of  which  the  differentiated 
product  can  be  manufactured.  If  there  is  not  such  a  preliminary  in- 
crease of  the  protoplasm,  then  the  differentiation  can  not  occur.  In 
order  that  perfection  of  the  adult  structure  should  be  attained,  it  is 
necessary  that  the  mere  undifferentiated  cells,  each  with  a  small  body 
of  protoplasm,  should  acquire  first  an  increased  amount  of  protoplasm, 
and  that  then  from  the  increased  protoplasm  should  be  taken  the 
material  to  result  in  differentiation,  in  specialization. 

An  undifferentiated  cell  performs  all  the  fundamental  functions  of 
life.  An  amoeba,  or  any  unicellular  organism  such  as  I  have  presented 
to  you  upon  the  screen,  does  everything  which  is  indispensable  to  life. 
It  takes  food;  it  forms  secretions  and  excretions;  its  activity  depends 
upon  chemical  alterations  going  on  in  the  food  in  the  interior  of  its 
body:  it  is  capable  of  sensation  and  of  locomotion.  It  is  probable 
that  every  living  cell  has  all  of  these  fundamental  properties  of  proto- 
plasm. When  a  cell  becomes  differentiated,  however,  though  it  does 
not  necessarily  give  up  any  of  its  vital  properties,  it  becomes  different 
from  other  cells  because  one  of  its  properties  is  made  conspicuous. 
And  in  order  to  acquire  that  conspicuousness,  that  excess  of  develop- 
ment of  one  function  of  the  cell,  a  modification  in  the  structure  is 
necessary.  The  apparatus  in  the  interior  of  a  cell  to  produce  the 
exaggeration  of  the  function  must  be  developed,  so  that  to  effect  the 
complex  physiological  machinery  of  the  adult  body,  this  differentiation, 
of  which  I  have  so  often  spoken,  is  indispensable.  A  nerve  cell  carries 
on  all  the  vital  functions,  but  it  has  in  addition  a  special  series  of 
modifications  of  its  protoplasm  which  enable  it  to  accomplish  the 
transmission  of  the  nervous  impulses  with  greater  efficiency  than  ordi- 
nary protoplasm  can  do,  probably  at  a  higher  speed  and  with  a  more 
perfect  adjustment  of  communication  between  the  various  parts  of 
the  body  than  is  possible  with  any  machinery  of  pure  protoplasm.     So 


AGE,    GROWTH   AND    DEATH  37 1 

too,  the  glands  have  cells  which  are  especially  capable  of  elaborating 
chemical  substances  which,  when  they  are  poured  out,  accomplish  the 
work  of  digestion,  for  instance.  But  these  cells  are  likewise  alive  in 
all  their  parts.  They  have  all  the  fundamental  vital  properties,  but 
there  is  this  tremendous  exaggeration  of  the  one  faculty,  and  that  in- 
volves an  alteration  so  great  in  the  protoplasm  that  we  can  see  it 
with  the  microscope ;  the  microscope  afEords  us  a  perfect  demonstration 
of  differentiation,  which  we  can  correlate  with  the  function. 

The  primary  object,  therefore,  of  all  differentiation  is  physiological. 
The  higher  organism,  with  its  complex  physiological  relations,  is  some- 
thing really  higher  in  structure  than  the  lower  organism.  The  term 
"  higher  "  in  biology  implies  a  much  more  complex  interrelation  of  the 
parts,  a  much  more  complex  relation  of  the  organism  to  the  outside 
world;  and  above  all  it  implies  in  the  highest  animals  a  complex 
intelligence  of  which  only  a  rudimentary  prophecy  exists  in  the  lowest 
forms  of  life,  possibly  scarcely  more  than  a  mere  sensation.  We  owe 
then  to  differentiation  our  faculties,  which  we  prize.  It  is  the  result  of 
differentiation  that  I  am  able  to  address  you  and  present  before  you 
the  thoughts  which  have  been  accumulated  as  the  result  of  the  studies 
of  many  years.  It  is  a  result  of  differentiation  that  you  have  such 
parts  that  you  not  only  hear  the  actual  sound  of  my  voice,  but 
interpret — at  least  I  hope  so — the  meaning  of  my  words  and  can 
understand  the  ideas  which  I  am  endeavoring  to  present  to  you.  If 
you  carry  away  something  from  these  lectures,  and  recall  it  at  some 
future  time,  that  also  will  be  a  result  of  the  differentiation  of  struc- 
ture; for  every  one  of  you  started  as  a  minute  germ,  consisting  of 
protoplasm  with  a  nucleus,  and  entirely  without  any  differentiation; 
and  by  a  process  so  complex  that  the  mystery  of  it  escapes  entirely  all 
our  powers  of  analysis,  those  parts  which  you  have  have  been  slowly 
and  secretly  fashioned.  We  have  approached  one  of  the  fundamental 
problems  of  existence.  When  we  talk  of  differentiation,  we  talk  of 
the  endowments  which  bring  us  into  relation  with  the  external  world — 
into  relations  with  our  kind,  and  which  make  our  internal  life  so 
complex,  a  complexity  which  in  itself  is  a  great  problem.  We  touch 
here  the  fundamental  mysteries  of  existence;  we  are  hovering  upon  the 
outskirts  of  our  human  conceptions.  We  are  not  yet  able  to  press 
beyond.  But  perhaps  the  time  may  come  when  the  limit  to  which  I 
can  now  bring  you  will  be  moved  farther  back,  and  some  of  the  things 
which  are  at  the  present  time  utterly  mysterious  and  incomprehensible 
to  us  will  be  comprehended  and  be  explicable  to  you. 

The  increase  of  the  protoplasm  is  then,  as  we  have  clearly  seen 
from  the  pictures,  the  mark  both  of  advancing  organization  and  of 
advancing  age.  It  is  certainly  somewhat  paradoxical  to  assert  that 
the  increase  of  the  protoplasm  is  a  sign  of  old  age,  a  sign  of  senescence, 
since  protoplasm  is  the  physical  basis  of  life.     It  undoubtedly  is  such, 


372  POPULAR    SCIENCE   MONTHLY 

and  we  slioiild  hardly  anticipate  that  its  increase  would  have  a  dele- 
terious effect.  But  such  is,  it  seems  to  me,  clearly  the  case.  But  it  is 
not  merely,  of  course,  a  question  of  the  increase  of  protoplasm  which 
we  must  bear  in  mind  in  estimating  the  cause  and  efEect,  but  also  the 
question  of  differentiation,  in  consequence  of  which  protoplasm  becomes 
something  else  and  difEerent  from  what  it  was  before.  This  altera- 
tion, then,  together  with  the  increase  of  the  protoplasm,  is  the  change 
which  in  all  parts  of  the  body  marks  the  passage  from  youth  to 
old  age. 

It  seems  to  me  not  going  at  all  too  far  to  say  that  the  increase  of 
protoplasm  is  a  fundamental  phenomenon.  I  wish  to  give  you  a  more 
precise  notion  of  this  increase;  and  I  am  glad  to  be  able  to  do  so  in 
consequence  of  a  research  carried  on  by  Professor  Eycleshymer  in  my 
laboratory  and  completed  by  him  afterwards  in  his  own  laboratory  at 
the  University  of  St.  Louis.  He  studied  the  development  of  the 
muscle  fibers  in  the  great  salamander,  known  scientifically  by  the  name 
of  Necturus.  These  muscle  fibers  are  somewhat  cylindrical  in  shape. 
Their  ends  can  be  accurately  determined  so  that  the  precise  length  of 
a  fiber  can  be  measured,  and  its  diameter  also.  Hence  the  total  volume 
of  a  fiber  may  be  calculated.  It  is  possible  also  to  measure  the  nuclei 
and  to  count  the  number  of  nuclei  in  a  fiber.  Thus  by  measuring  the 
diameter  and  length  of  the  fiber,  and  then  estimating  the  number  and 
the  diameters  of  the  nuclei,  we  can  calculate  the  proportions.  As  a 
matter  of  fact,  the  nuclei  remain  nearly  constant  in  volume,  not  really 
quite  so,  but  sufficiently  constant  to  serve  as  a  basis  of  measurement. 
Dr.  Eycleshymer  found  that  when  a  Necturus  had  a  length  of  eight 
millimeters,  it  possessed,  for  each  nucleus  in  its  muscle  fiber,  2,737 
units  of  protoplasm,  but  when  it  was  seventeen  millimeters,  it  possessed 
for  each  nucleus  4,318  units  per  nucleus;  at  twenty-six  millimeters, 
8,473  units;  and  in  the  adult,  which  measures  approximately  230 
millimeters,  it  has  22,379  units  per  nucleus.  In  other  Avords,  as  a 
salamander  passes  from  the  eight-millimeter  condition,  when  the  de- 
velopment of  its  muscle  fibers  is  just  fairly  begun,  up  to  the  adult  state, 
Avhen  the  differentiation  of  the  muscle  fibers  has  been  completed,  it 
increases  the  proportion  of  protoplasmic  substance  and  protoplasmic 
derivatives  from  2,700  to  22,300  per  nucleus.  I  give  round  numbers. 
The  increase  is  approximately  eightfold.  There  is  in  the  adult  in  the 
muscle  fiber  eight  times  as  much  i^rotoplasmic  substance  in  propor- 
tion to  the  nucleus  as  there  was  at  the  start  of  development  when  the 
muscle  fiber  could  first  be  clearly  recognized  as  such.  This  is  an 
accurate  measure  and  'gives  us  a  good  idea  of  the  general  law  of  proto- 
plasmic increase.  It  is  the  only  instance,  I  yet  know  of,  in  which  we 
have  an  accurate  measure  and  can  give  quantitative  values,  though  we 
do  know  that  there  is  a  more  or  less  similar  increase  occurring  in  per- 
haps every  tissue  of  the  body. 


AGE,    GROWTH   AND    DEATH  373 

While  the  increase  of  the  protoplasm  is  going  on,  we  find  that  there 
is  an  advance  in  the  structure,  in  the  differentiation.  Xow  you  may 
recall  what  I  have  mentioned  earlier  in  this  lecture,  the  further  funda- 
mental fact  that  the  loss  in  the  rate  of  growth  is  greatest  in  the  young, 
least  in  the  old,  and  that  as  we  go  back  from  old  age  towards  youth, 
and  then  into  the  embryonic  period,  we  find  an  ever-increasing  power 
of  growth,  but  that  it  is  during  the  embryonic  period  that  the  loss  of  the 
power  of  growth  is  greatest.  It  is  to  the  embryonic  period,  therefore, 
that  I  have  turned  in  order  to  ascertain  whether  the  rate  of  differentia- 
tion shows  a  similar  relation  in  the  development  of  the  organism. 

We  have  a  large  series  of  microscopic  preparations  of  rabbit  embryos 
in  the  embryological  laboratory  of  the  Harvard  Medical  School. 
Utilizing  these,  I  found  that  at  seven  or  eight  days  of  development  there 
is  scarcely  a  trace  of  differentiation.  The  cells  are  in  the  condition  of 
those  which  I  showed  to  you  earlier  in  the  lecture  upon  the  screen.  At 
sixteen  and  a  half  days,  a  stage  of  development  of  which  I  have  some 
good  preparations,  I  found  that  a  great  deal  had  been  accomplished. 
x4.t  seven  days  there  was  no  brain,  there  was  no  spinal  cord,  nothing 
that  could  possibly  be  called  skin  or  muscle,  or  intestine  or  heart 
Xone  of  those  things  were  yet  produced.  But  at  sixteen  and  one  half — 
in  other  words,  after  a  very  brief  period  indeed — only  nine  days  of  the 
whole  life  of  the  animal — there  have  arisen  from  this  inchoate  begin- 
ning all  the  principal  organs  of  the  body.  The  brain  is  there,  divided 
up  into  its  principal  fundamental  parts;  the  spinal  cord  has  its  nerves 
in  connection  with  the  various  parts  of  the  body;  there  is  a  trace  of 
the  skeletal  element ;  the  stomach,  the  liver,  the  pancreas,  the  intestines, 
are  all  present  and  well  defined ;  the  heart  is  a  large  and  beating  organ, 
amply  supplied  with  blood,  connected  with  vessels,  which  carry  out  and 
bring  back  the  blood  and  are  all  far  along  in  their  development. 
Equally  instructive  is  the  microscopic  examination,  for  we  can  see  that 
the  cells  themselves  have  been  changed.  ISTot  only  have  the  great 
organs  been  mapped  out  in  this  brief  period,  but  the  cells  which  belong 
to  them  have  for  each  organ  acquired  a  characteristic  quality.  In  the 
brain  there  are  nerve  cells  with  their  long  processes  to  carry  the  im» 
pulse  in;  the  single  process  (axon)  to  carry  it  out.  The  glands  in  the 
stomach  have  the  cells  which  arc  to  build  them  already  there.  Tlie 
muscles  which  are  to  move  the  stomach  are  beginning  to  appear  as  cells 
of  a  special  form.  Nerve  fibers  extend  down  into  the  gastric  region 
and  to  the  various  distant  organs  of  the  body.  Muscle  fibers  can  be 
recognized  along  the  back  and  in  the  limbs,  and  so  in  every  part  of 
the  Vjody  we  can  detect  cells  already  far  advanced  in  their  develop- 
ment. It  is  not  certainly  too  much  to  say  that  in  the  brief  period  of 
these  nine  days  fully  as  much  differentiation  has  been  accomplished  as 
is  accomplished  during  the  entire  remainder  of  the  life  of  the  animal. 
We  do  not,  at  present  at  least,  possess  any  method  of  measuring  dif- 


374  POPULAR    SCIENCE   MONTHLY 

ferentiation,  which  enables  us  to  state  it  numerically,  but  no  one  who 
is  familiar  with  these  matters  and  observes  the  structure,  as  I  have 
myself  observed  it,  would  hesitate  for  a  moment,  it  seems  to  me,  to 
decide  that  my  assertion  is  perfectly  within  the  bounds  of  truth,  that 
within  a  period  of  nine  days,  half  of  the  entire  differentiation  which  is 
to  occur  in  the  whole  life  of  the  rabbit  has  been  completed.  We  must 
from  this  conclude  that  the  rate  of  differentiation  is  very  rapid  at  first 
and  afterwards  declines,  and  as  we  compare  the  different  stages  of 
development  we  can  see  readily  that  this  is  the  case.  The  progress  in 
the  additional  development  in  the  rabbit  from  sixteen  and  one  half 
days  up  to  the  time  of  its  birth  is  far  greater  than  the  progress  which 
occurs  after  birth.  We  find,  moreover,  in  the  study  of  these  embryonic 
conditions,  some  instructive  things,  for  in  certain  parts  of  the  body  the 
process  of  differentiation  hurries  along,  and  as  the  cells  are  differ- 
entiated their  power  of  growth,  to  a  large  extent,  is  stopped.  On  the 
other  hand,  there  are  various  provisions  in  the  developing  animal  for 
keeping  back  certain  cells,  allowing  them  to  remain  in  the  young  state. 
Such  cells  may  afterward  differentiate. 

From  all  that  has  been  said  it  seems  to  me  legitimate  to  conclude 
that  there  is  an  intimate  correlation  between  the  rate  of  differentia- 
tion and  the  rate  of  growth.  I  am  inclined  to  go  the  one  step  farther, 
and  bring  them  into  the  relation  of  cause  and  effect ;  and  I  present  to 
you  as  the  main  general  conclusion  of  this  first  part  of  our  series  of 
lectures,  the  conception  that  the  growth  and  differentiation  of  the 
-protoplasm  are  the  cause  of  the  loss  of  the  poiver  of  growth.  Now  if 
cells  become  old  as  their  protoplasm  increases  and  becomes  differ- 
entiated, we  should  expect  to  find  that  there  would  be  a  provision  for 
the  production  of  young  cells.  It  is  rather  mortifying  to  reflect  that 
the  simple  conception  which  I  have  now  to  express  to  you,  although  it 
lay  close  at  hand,  failed  to  combine  itself  in  my  mind  for  many  years 
with  the  conception  of  the  process  of  senescence  as  I  have  just  described 
it  to  you.  It  is  somewhat,  it  seems  to  me,  like  two  acquaintances  of 
mine  who  lived  long  side  by  side,  seeing  one  another  frequently  until 
they  were  fairly  past  the  period  of  youth,  when  their  attachment  be- 
came very  close  and  by  a  sacrament  they  were  permanently  joined 
together.  So  in  the  minds  of  men  often  two  ideas  lie  side  by  side  which 
ought  to  be  married  to  one  another,  and  there  is  no  one  ready,  so  dull 
is  the  owner  of  the  mind,  to  pronounce  the  sacramental  words  which 
shall  join  them,  and  the  rite  long  remains  unperformed,  and  when  at 
last  such  neighbor  ideas,  which  naturally  should  be  united  in  close 
companionship,  are  brought  together  and  made,  as  it  were,  into  one,  we 
are  astonished  that  the  inevitableness  of  the  union  had  not  obtained 
our  notice  before,  it  is  so  very  obvious.  And  so  in  regard  to  the  con- 
ception of  what  constitutes  the  restoration  of  the  young  state,  I  have 
only  this  excuse  to  offer,  which  I  have  indicated  to  you,  that  even  the 


AGE,    GROWTH   AND    DEATH 


375 


natural  thought  fails  to  occur  to  us.     We  are  very  dull  even  if  we  are 
scientific. 

The  pictures  now  before  you  represent  certain  early  stages  in  the 
progress  of  development  of  a  mammal  by  the  name  of  Tarsius,  a 
creature  related  to  the  lemurs.     The  various  figures  illustrate  the  multi- 


FlG.  49. 


Tarsius  speclabile.    Sections  of  Three  Ova  in  very  Early  Stages. 
cleavage  ;  2,  cleavage  into  four  cells ;  3,  multicellular  stage. 


1,  before 


plication  of  the  cells.  That  which  I  wish  to  call  your  attention  to 
can  be  well  demonstrated  by  the  comparison  of  the  first  figure,  in 
which  there  is  a  single  nucleus,  with  the  figure  at  this  point  having  a 
number  of  nuclei.  Both  figures  represent  the  very  earliest  stages  of 
development  and  show  the  full  size  of  the  whole  germ,  which  is  about 
the  same  in  the  two  stages.  The  total  amount  of  living  material  has 
not-  changed  essentially,  but  evidently  there  has  occurred  a  marked 
increase  of  the  nuclear  substance.  The  nuclei  have  in  the  right-hand 
figure  multiplied  in  number  and  their  combined  volume  is  much  greater 
than  the  total  volume  of  the  single  nucleus  in  the  left-hand  figure. 

We  can  get  a  further  notion  of  the  nuclear  increase  by  studying  the 
very  early  development  of  a  salamander.  Here  upon  the  screen  is  the 
Q^g  of  a  salamander.  It  represents  really  but  a  single  cell.  It  then 
divides  into  two  cells;  each  of  those  cells  has  a  nucleus  which  we  can 
not  see  because  these  pictures  are  taken  from  the  living  egg,  and  the 
living  egg  is  not  transparent.  Here  it  is  dividing  into  four,  here  the 
upper  portion  of  the  four  cells  has  been  split  off,  and  we  have  seven 
cells  showing  in  the  figure,  and  an  eighth  on  the  back.  Here  the 
number  of  cells  has  increased  very  much,  and  as  you  view  these  figures 
you  will  notice  that  they  look  very  much  indeed  like  oranges  divided 
into  segments.  It  seems,  in  fact,  as  if  this  egg,  which  was  spherical  in 
form,  were  being  divided  up  into  a  certain  number  of  segments.  The 
process  was  first  observed  in  the  eggs  of  some  of  the  amphibia,  frogs, 
toads  and  salamanders,  and  it  was  therefore  called  segmentation,  be- 
cause it  was  not  known  at  that  time  what  the  process  really  meant. 
We  have  then  before  us  an  ovum  and  a  series  of  stages  of  the  segmenta- 
tion of  the  ovum,  and  the  result  of  tliat  segmentation  is  to  produce  an 


376 


POPULAR    SCIENCE    MONTHLY 


ever-increasing  number  of  cells  which,  in  the  last  of  the  figures  upon 
the  screen,  have  become  so  numerous  that  we  are  no  longer  able  to 
readily  count  them.  Every  one  of  these  cells  has  its  own  nucleus. 
When  the  process  of  segmentation  is  complete  and  reaches  its  final 
limit,  we  then  see,  if  we  examine  that  stage  of  development,  cells  of  the 
young  type,  such  as  I  have  described  to  you,  in  which  there  is  a 
nucleus  with  a  small  amount  of  protoplasm  about  each  nucleus.  It 
seems  to  me,  therefore — and  this  is  a  new  interpretation  which  I  present 


Fig. 


Ainblyslomum  punciatum.    Progressive  Segmentation  of  the  Ovdm. 
1.  unsegmented  ovum  ;  9,  advanced  segmentation 


to  you — that  the  process  of  segmentation  of  the  ovum,  with  which  the 
development  of  all  the  animals  of  the  higher  type  invariably  begins, 
is  really  the  process  of  producing  young  cells.  It  is  the  process  of 
rejuvenation.  There  is  not  any  considerable  growth  of  the  living  pro- 
toplasmic material  of  these  eggs,  and  at  the  final  stage  the  total  volume 
of  the  egg  is  scarcely  bigger  than  before;  and  such  increased  volume 
as  has  occurred  has  been  due  to  the  absorption  of  some  of  the  surround- 
ing water.     In  many  animals  not  even  this  increase  by  the  absorption 


AGE,    GROWTH   AND    DEATH  377 

of  water  takes  place.  During  the  segmentation  of  the  ovum  the  condi- 
tion of  things  has  been  reversed  so  far  as  the  proportions  of  nucleus  and 
protoplasm  are  concerned.  We  have  nucleus  produced,  so  to  speak,  to 
excess.  The  nuclear  substance  is  increased  during  this  first  phase  of 
development. 

N'aturally,  as  we  embryologists  looked  upon  these  things  in  earlier 
days  and  thought  of  the  progress  of  development,  we  conceived  of  the 
earlier  stage  as  younger,  and  of  the  ovum  as  being  the  youngest  stage 
of  all,  a  conception  which  in  terms  of  time  is  obviously  correct,  but 
as  regards  the  nature  of  the  development,  it  seems  to  me  clearly,  is  not 
correct.  The  ovum  is  a  cell  derived  from  the  parent  body,  fertilized  by 
the  male  element,  and  presenting  the  old  state  to  us,  the  state  in  which 
there  is  an  excessive  amount  of  protoplasm  in  proportion  to  the  nucleus ; 
and  in  order  to  get  anything  which  is  young,  a  process  of  rejuvena- 
tion is  necessary,  and  that  rejuvenation  is  the  first  thing  to  be  done 
in  development.  The  nuclei  multiply;  they  multiply  at  the  expense 
of  the  protoplasm.  They  take  food  from  the  material  which  is  stored 
up  in  the  ovum,  nourish  themselves  by  it,  grow  and  multiply  until  they 
become  the  dominant  part  in  the  structure.  Then  begins  the  other 
change;  the  protoplasm  slowly  proceeds  to  grow,  and  as  it  grows,  dif- 
ferentiation follows,  and  so  the  cycle  is  completed.  Whether  other 
naturalists  will  be  inclined  to  accept  this  conception  that  the  process  of 
the  segmentation  of  the  ovum  is  that  which  we  must  call  rejuvenation 
or  not,  I  can  not  say,  for  the  matter  has  as  yet  been  very  little  discussed, 
but  you  will  see  that  as  a  theory  it  hangs  well  together.  We  have 
first  an  explanation  of  the  process  of  the  production  of  the  young 
material,  and  out  of  that  young  material  the  fashioning  of  the  embryo. 
The  cycle  of  life  has  two  phases,  an  early  brief  one,  during  which  the 
young  material  is  produced,  then  the  later  and  prolonged  one,  in 
which  the  process  of  differentiation  goes  on,  and  that  which  was  young, 
through  a  prolonged  senescence,  becomes  old.  I  believe  these  are  the 
alternating  phases  of  life,  and  that  as  we  define  senescence  as  an  in- 
crease and  differentiation  of  the  protoplasm,  so  we  must  define  rejuvena- 
tion as  an  increase  of  the  nuclear  material.  The  alternation  of  phases 
is  due  to  the  alternation  in  the  proportions  of  nucleus  and  protoplasm. 

In  the  next  lecture  I  shall  be  able  to  convince  you,  I  hope,  that  this 
conception  of  the  relation  of  the  power  of  growth  to  the  proportion  of 
nucleus  and  protoplasm  enables  us  to  understand  various  problems  of 
development,  certain  possibilities  of  regeneration  and  reconstruction  of 
lost  parts,  and  that  it  also  leads  us  naturally  forward  to  the  considera- 
tion of  the  problem  of  death  as  it  is  now  viewed  by  biologists,  so  that 
our  next  lecture  will  be  upon  the  subject  of  regeneration  and  death,  the 
natural  topics  to  follow  after  to-night's  discussion. 


455 


AGE,    GROWTH    AND    DEATH 


THE  PKOBLEM  OF  AGE,  GEOWTH  AND  DEATH 


By  CHARLKS  SEDGWICK  MINOT,  LL.D.,  D.Sc. 

JAMES  STILLMAN   PROFESSOR   OF  COMPARATIVE  ANATOMY,   HARVARD   MEDICAL  SCHOOL 

Y.  Eegeneration  and  Death 
Ladies  and  Gentlemen :  In  the  last  lecture  I  treated  the  conception 
I  had  formed  of  the  processes  of  regeneration  and  told  yon  that  I 
looked  upon  the  change  which  occurred  first  in  the  developing  germ 
as  one  of  rejuvenation.  The  process  has  for  its  technical  name  the 
segmentation  of  the  ovum.  The  appearance  of  this  segmentation  proc- 
ess was  illustrated  to  you  by  the  pictures  thrown  upon  the  screen. 
Cytomorphosis  is  a  term  which  we  have  frequently  used  in  the  course 


Fig.  si.    The  Segmentation  of  the  Ovum  of  Amblystoma  punctatum,  to  show  the 
earliest  phases  of  development  in  the  egg  of  a  newt.    After  A.  C.  Eycleshymer. 


POPULAR   SCIENCE   MONTHLY 


456 


of  these  lectures,  and  I  have  led  yon,  I  hope,  to  the  appreciation  of  the 
idea  that  in  cjtomorphosis  we  have  at  least  a  part  of  the  explanation  of 
old  age.  We  have  learned  that  the  young  cells  which  are  produced  by 
the  segmentation  of  the  ovum  in  the  body  in  large  part  changed  into 
old  cells,  and  also  that  old  cells  can  not  go  back  in  their  development 
and  again  become  young;  so  that  one  might  easily  be  led  to  the  sus- 
picion that  there  could  be  no  possible  new  young,  a  conclusion  obviously 
absurd,   for  there  is  a  constant  renewal   of  the  generations.       Some 


Fig.  52.    The  Se'jmentateon  ok  the  Ovum  of  Plaanrhis,  to  show  the  earliest  phases  of 
development  of  the  egg  of  a  pond  snail.    Alter  Carl  Rabl. 


device,  therefore,  must  exist  by  which  that  which  is  young  is  perpetu- 
ated, for  that  which  is  old  can  not  again  become  young,  and  of  that 
device  I  should  like  to  say  something  this  evening. 

As  a  preliminary  to  the  discussion  of  this  interesting  phenomenon, 
it  is  necessary  to  say  a  few  more  words  in  regard  to  the  nuclei.  You 
recall  that  the  units,  out  of  which  the  body  is  constructed,  the  cells, 
consist  each  of  a  little  mass  of  protoplasm  with  a  central  body  called 
the  nucleus;  and  you  will,  I  hope,  recall  that  the  increase  of  the  proto- 
plasm and  the  subsequent  differentiation  of  the  cell  we  looked  upon  as 
the  cause  of  old  age,  and  the  increase  of  the  nucleus  as  the  cause  of 
youth,  of  rejuvenation.      In  addition  to  what  has  been  said  concerning 


457 


AGE,    GROWTH   AND    DEATH 


the  size  of  the  nucleus,  some  further  explanation  is  necessary,  and  that 
can  best  be  given  with  the  aid  of  some  illustrations  upon  the  screen. 
The  first  of  the  pictures  will,  I  hope,  serve  to  recall  to  your  minds  what 
I  said  in  regard  to  the  process  of  the  segmentation  of  the  ovum.  Here 
is  an  ovum,  No.  1,  a  single  cell,  but  relatively  of  enormous  size,  the 
ovum  or  germ  of  a  newt,  and  the  plate  illustrates  to  us  the  gradual 
process  of  division  of  the  original  single  cell  into  a  number  of  distinct 
cells,  and  each  of  these  we  call  a  segment,  and  the  formation  of  them, 
segmentation,  a  name  which  we  keep  from  the  olden  time  when  the 
process  was  first  observed  by  some  French  investigators,  because  it  is 
so  descriptive  of  the  appearance  presented  to  the  eye  by  the  changes 
which  are  going  on.  Were  we  to  name  the  process  now  we  should  cer- 
tainly call  it  a  process  of  cell  production. 

The  next  of  our  pictures  shows  us  the  eggs  of  a  common  snail,  the 
Planorbis,  a  little  fresh-water  snail,  the  coils  of  which  lie  flat  in  one 
plane — ^lience  its  name.  'No.  1  is  the  original  germ;  No.  2  shows  it 
about  to  divide  into  two;  No.  3  is  a  side  view;  No.  4  a  top  view  of 
the  ovum  with  tAvo  segments;  No.  5  is  cleft  into  four  segments;  No.  6 
into  eight.  Nos.  7  and  8  illustrate  the  further  progress  of  the  cell 
multijDlication ;  No.   9  represents  the  under  side  of  the  same  egg  of 


/  2  J 

Fig.  53.    Three  Sections  theough  the  Segmenting  Ova  of  a  Mammal,  Tarsius 

spectabile. 

which  the  top  is  figured  as  No.  8.  The  number  of  cells  (segments)  is 
thus  constantly  increasing  and  already  it  is  evident  that  they  have 
become  somewhat  unlike  in  character.  Were  the  picture  still  further 
magnified,  we  could  see  that  in  these  cells  a  change  is  going  on  in  the 
nuclei  which,  however,  I  can  better  demonstrate  to  you  by  means  of  the 
following  picture,  one  which  we  saw  in  the  last  lecture.  These  are 
sections  through  the  early  developing  germ  of  a  mammal  named  Tar- 
sius spectabile.  It  is  a  creature  nearly  related  to  the  lemurs,  having 
a  special  interest  to  naturalists,  owing  to  the  fact  that  in  its  early 
development  it  offers  features  of  resemblance  to  man  which  are  very 
striking  and  instructive.     The  plate  is  from  a  series  of  drawings  made 


POPULAR    SCIENCE   MONTHLY  458 

under  the  direction  of  Professor  Hubrecht,  the  principal  student  of  the 
development  of  this  type  of  animal.  Here  (Xo.  1)  we  can  see  an  early- 
stage  in  which  the  germ  consists  of  but  a  single  cell,  and  at  this  point 
is  the  nucleus.  Xote  its  size  and  then  compare  it  with  the  nuclei  in 
Xos.  2  and  3  in  which  several  of  these  cells,  as  they  appear  in  a  section, 
are  represented.  The  cells  themselves  are  now  smaller  because  thoy 
have  multiplied  by  the  division  of  the  original  germ,  but  the  nuclei  in 
them  are  likewise  smaller.  And  in  the  older  stage,  jSTo.  3,  where  the 
nimiber  of  cells  has  begun  still  further  to  increase,  we  see  that  there  is 
another  and  more  marked  reduction  in  the  size  of  the  nuclei.  Contrast 
the  single  nucleus  of  the  early  stage  with  the  small  nuclei  of  the  later 
one,  and  notice  how  very  striking  is  the  change  in  the  size.  Thus  dur- 
ing the  early  development  of  the  individual,  and  it  seems  to  be  true  of 
all  animals,  we  find  that  there  is  an  actual  rapid  reduction  in  the  size 
of  the  nucleus.  As  we  have  learned  that  the  proportion  of  the  nucleus 
and  the  protoplasm  is  so  important,  we  must  attribute  to  this  alteration 
in  the  dimensions  of  the  nucleus  great  significance. 

We  have  next  a  series  of  figures  which  have  interested  me  very 
much  and  which  I  only  recently  secured  as  the  result  of  studies  I  have 
been  making  in  my  own  laboratory  at  the  Harvard  Medical  School. 
These  pictures  are  now  shown  publicly  for  the  first  time,  and  record  a 
fact  which,  so  far  as  I  know,  has  never  yet  been  clearly  noted  and 
recognized  as  important  by  any  investigator.  The  four  figures  at  tlie 
top  represent  four  single  nuclei  taken  from  different  parts  of  a  rabbit 
seven  and  one  half  days  after  the  commencement  of  its  development. 
The  second  set  of  figures,  5,  6,  7  and  8,  show  nuclei  from  different 
characteristic  parts  of  a  rabbit  embryo  of  ten  days.  Note,  please,  the 
size  of  these  nuclei,  the  curious  network  of  threads  in  their  interior 
and  the  existence,  generally  more  or  less  in  a  central  position,  of  a  mass 
of  material  which  stands  out  conspicuously  and  represents  a  condensa- 
tion of  the  nuclear  stuff  at  that  particular  point.  Such  a  central  body 
is  highly  characteristic  of  these  early  stages.  Xext  we  come  in  the 
series  of  figures,  from  9  to  20,  stretching  across  the  screen  in  twO' 
lines,  to  a  rabbit  embryo  of  twelve  and  one  half  days.  Instead  of 
having  nuclei  of  large  size  we  have  now  nuclei  which  are  obviously 
small.  Instead  of  having  nuclei  which  are  more  or  less  alike  in  ap- 
pearance, we  have  now  nuclei  of  great  diversity.  Every  one  of  these 
figures,  as  you  will  readily  see  if  you  run  your  eye  along  from  one  end 
of  the  lines  to  the  other,  has  a  distinctive  character  of  its  own.  In  this 
period,  then,  of  two  and  one  half  days,  there  has  been  a  revolution  in 
the  character  of  the  nuclei  of  the  developing  embryo.  Where  before 
the  nuclei  were  alike,  now  they  have  become  unlike.  Two  of  these  I 
should  like  especially  to  call  your  attention  to,  because  they  are  the 
nuclei  of  the  nerve  cells — this  one,  Xo.  11,  from  tl)C  spinal  cord  and 
the  right-hand  one,  No.  10,  from  the  cluster  of  nerve  cells  upon  the 


459 


AGE,    GROWTH   AND    DEATH 


Fig.  54.  Nuclei  FROM  Rabbit  Embryos.  1-4,  age,  seven  and  one  half  days.  5-8,  age,  ten 
days.  9-20,  age,  twelve  and  one  half  days.  2]-33,  age,  sixteen  and  one  half  days. 
1,  ectoderm  ;  2,  mesoderm;  3,  entoderm;  4,  Hensen's  knot;  5,  entoderm;  6,  mesen- 
chyma;  7,  entoderm;  8,  medullary  groove;  9,  ectoderm;  10,  large  motor  neurone;  11.  spinal 
ganglion;  12,  mesenchyma ;  13,  cartilage  ;  14,  Wolfiian  body;  15,  kidney;  16,  striated  muscle  ; 
17,  heart  muscle;  18,  esophageal  entoderm;  19,  tracheal  entoderm;  20,  liver;  21,  --ntiiderm;  22, 
motor  neurone;  23,  spinal  ganglion  ;  24,  dermis  ;  25,  hypodermis  ;  26.  cartilage  ;  27,  28,  Wolffian 
tubules;  29,  pelvis  of  kidney  ;  30,  heart  muscle;  31,  esophageal  entoderm;  32,  tracheal  ento- 
derm.   After  A.  A.  W.  Hubrecht. 

root  of  a  spinal  nerve.  Finally,  we  have  the  series  of  figures  from  a 
rabbit  of  sixteen  and  one  half  days  represented  in  the  two  lower  rows, 
21  to  33.  In  these,  if  yon  will  leave  aside  from  consideration  for  the 
moment  22  and  23,  which  are  obviously  of  a  different  size,  all  are  now 
smaller  than  they  were  at  twelve  and  one  half  days.  Every  one  of  the 
nuclei  here  represented  is  characteristic.  We  have  here,  for  instance, 
nuclei  of  the  excretory  organ;  a  nucleus  of  the  connective  tissue; 
we  have  nuclei  from  the  lining  of  the  wind-pipe  and  the  lining 
of  the  gullet.  Every  one  of  them  differs  from  every  one  of  the 
others  pictured.  But  if  we  had  drawings  of  a  number  of  nuclei 
from  the  same  part  of  the  body  and  same  kind  of  tissue,  we  should 
see  that  they  would  be  essentially  similar.  We  learn  then  that 
there  is  acquired  a  great  diversity  in  the  structure  of  the  nuclei 
as  well  as  in  that  of  the  protoplasm,  of  which  we  have  seen  so 
many  examples  in  the  previous  lectures.  You  will  recall,  that  as 
regards  the  size  of  cells  the  nerve  cells  present  a  noteworthy  excep- 


POPULAR    SCIENCE   MONTHLY  460 

tion  in  that  they  differ  according  to  the  size  of  the  animal;  and 
their  nuclei  differ  also,  for  as  the  cells  become  big  the  nuclei  grow 
likewise.  Here  are  nerve-cell  nuclei  in  the  rabbit  of  twelve  and 
one  half  days  not  differing  in  their  dimensions  essentially  from  the 
nuclei  of  other  types,  but  in  the  two  lower  figures,  22  and  23,  we 
see  nuclei  corresioonding  to  the  cells  of  the  rabbit  at  sixteen  and 
one  half  days.  These  cells  have  begun  to  enlarge,  to  assume  the 
greater  dimensions  of  the  nerve  cells,  which  is  characteristic  of  the 
rabbit;  and  accompanying  the  enlargement  of  the  cells  there  has  been 
an  expansion  of  the  nuclei  also.  But  this  does  not  affect,  as  you  will 
readily  see  by  the  pictures  upon  the  screen,  the  nuclei  of  any  other  sort 
of  tissue,  the  nuclei  of  any  other  organ  of  the  body. 

We  must  therefore  add  to  our  conceptions  in  regard  to  the  relations 
of  the  nucleus  and  protoplasm,  as  quantitatively  expressed,  this  further 
notion  that  there  is  during  the  early  period  of  development  an  actual 
reduction  in  the  size  of  the  nucleus.  When  this  reduction  has  taken 
place  it  is  of  course  evident  to  any  one  at  all  acquainted  with  the  prin- 
ciples of  cytology  that  the  cells  are  in  a  very  different  state  from  what 
they  were  in  before.  They  are  no  longer  such  cells  as  they  were  when 
the  nucleus  was  large,  and  the  nuclei  in  the  different  parts  of  the  body 
alike  in  character.  Here  the  relations  are  fundamentally  changed. 
We  do  not  find  that  these  nuclei  ever  get  back  from  the  complex  variety 
of  organization  which  they  present  to  us  in  later  stages  to  the  earlier 
condition  when  they  were  all  alike ;  yet  only  with  cells  of  this  uniform 
sort  can  development  begin.  We  should  therefore,  if  we  reasoned  only 
from  the  data  which  I  have  thus  far  presented  to  you,  come  to  the  con- 
clusion that  reproduction  would  be  impossible,  that  the  cells  of  the 
body,  having  been  so  changed,  as  we  have  seen,  are  no  longer  capable  of 
returning  backwards  along  the  path  they  have  journeyed ;  they  can  only 
remain  where  they  are,  or  go  yet  further  onward  in  the  career  of  cyto- 
morphosis.  Nature,  however,  has  met  this  difficulty  by  a  way  which 
we  have  only  recently  discovered.  We  are  not  yet  sure  that  the  way 
we  have  discovered  is  the  only  way,  that  it  is  the  universal  method  in 
the  case  of  all  animals  for  accomplishing  the  purpose.  The  discovery 
of  this  method  of  providing  for  the  perpetuation  of  youthfulness  from 
one  generation  to  another,  the  youthfulness  of  the  cell  of  man,  is  due 
to  the  investigations  of  Professor  Nussbaum,  of  Bonn.  The  theory, 
which  he  put  forward,  has  been  verified  by  subsequent  examinations 
and  investigation,  and  confirmed,  I  am  glad  to  say,  in  part  by  some 
very  interesting  and  careful  observations  which  have  been  made  here 
in  Boston.  Perhaps  the  very  best  confirmation  of  all  is  the  recent 
extension  of  our  knowledge  in  regard  to  this  theory  which  comes  from 
the  investigations  of  Dr.  B.  M.  Allen,  made  at  Madison,  on  the  process 
as  we  find  it  in  the  developing  turtle.  It  is  really  essentially  a  very 
simple  thing.     Nature  seems  to  take  some  of  the  cells  which  are  in  the 


46 1  AGE,    GROWTH   AND    DEATH 

primitive  condition,  with  tlie  protoplasm  still  undifferentiated  and  the 
nucleus  of  the.  embryonic  or  simple  organization,  and  hold  them  apart 
from  the  rest  of  the  body,  not  separating  them  so  that  they  come  off 
and  leave  the  body,  but  so  that  they  have  a  different  history;  so  that 
they  escape  the  change  which  the  other  cells  of  the  body  must  pass 
through.      These  cells  of  a  simpler  character  are  gathered  together, 
kept  asunder,  and  not  allowed  to  progress  in  the  development  of  all  the 
cells  which  form  the  body  proper.     We  have  learned,  for  instance,  that 
in  the  development  of  the  dog-fish  very  early,  before  any  organs  exist, 
cells  are  formed  into  a  cluster.      They  lie  by  themselves,  are  easily 
recognized  under  the  microscope,  and  they  have  obviously  the  primitive 
character  which  I  have  endeavored  to  explain  to  you.      And  they  re- 
main such.      Meanwhile  as  development  progresses,  all  the  remaining 
cells — all  those  not  part  of  these  clusters,  pursue  their  proper  careers, 
become  differentiated;  but  the  cells  in  the  clusters  do  not  change  for 
a  long  period.      Later  as  the  organs  become   differentiated,   we   can 
recognize  in  the  direct  descendants  of  these  cells,  which  have  been 
traced  from  stage  to  stage  so  that  their  history  is  known  with  certainty, 
those  cells  which  in  the  adult  we  call  the  germ  cells,  and  which  are  to 
serve  for  the  reproduction  of  the  species.      These  cells  are  set  apart  at 
all  periods.      They  represent  germinal  matter  which  is  withheld  from 
the  metamorphosis  which  the  rest  of  the  body  undergoes.      They  have 
a  continuous  history.      Hence  we  bestow  upon  this  method,  under  the 
conception  that  it  is  applied  to  secure  propagation  of  the  species,  the 
term- — theory  of  germinal  continuity.      It  is  the  theory  of  hereditary 
transmission  which  I  think  is  now  universally  held  by  all  competent 
biologists.      Our  study  of  nuclei  and  of  their  relations  to  protoplasm 
serves  to  clear  up  in  our  minds,  it  seems  to  me,  to  some  degree  at  least, 
the  necessity  which  really  exists  for  this  device  of  germinal  continuity, 
of  the  setting  apart  of  certain  cells  of  the  rejuvenating  sort,  of  the 
young  sort,  of  the  embryonic  type  (the  term  you  apply  to  them  matters 
little),  which  cells  are  those  used  to  produce  the  new  offspring  of  the 
next  generation.      All  this,  of  course,  fits  perfectly  with  the  doctrine 
which  I  have  been  telling  you  of  again  and  again  in  this  course  of 
lectures,  that  the  progress  of  differentiation  is  always  in  one  direction 
and  ends  in  the  production  of  structure  which,  if  it  is  pursued  to  its 
legitimate  terminus,  results  in  the  degeneration  and  death  of  the  cell. 
Obviously  such  a  set  of  changes  as  I  have  thus  indicated  can  not  produce 
the  sort  of  a  cell  which  is  necessary  for  reproduction. 

I  wish  there  were  time  to  enter  more  fully  into  this  question  of  the 
size  of  nuclei,  for  there  is  much  which  might  be  said  concerning  it. 
This  much  more,  however,  ought  to  be  said  to  you — that  the  problem  of 
the  size  of  nuclei  is  by  no  means  a  simple  one.  It  has  been  found,  for 
instance,  in  the  experiments  made  upon  some  of  the  simple  algge,  the 
so-called  Spirogyra,  which  every  elementary  student  of  botany  probably 


POPULAR    SCIENCE   MONTHLY  462 

has  looked  at  in  the  laboratory,  that  by  certain  artificial  conditions,  as 
made  in  the  experiments  of  Professor  Gerassimow,  'the  size  of  the 
nucleus  can  be  changed  in  the  cells,  and  when  the  size  of  the  nucleus 
is  changed,  the  size  of  the  cell  alters  also.  And  again,  we  know  that 
the  nucleus  provides  certain  chemical  supplies  for  the  life  and  func- 
tioning of  the  cells.  This  is  very  strikingly  the  case,  for  instance,  in 
regard  to  the  cells  which  secrete.  These,  when  they  give  ofE  the  ma- 
terial which  they  have  accumulated  in  their  protoplasm  as  a  prepara- 
tion for  the  act  of  secretion,  are  found  not  only  to  reduce  the  bulk  of 
their  protoplasmic  bodies,  but  the  bulk  of  the  nuclei  as  well.  And  we 
know  again  that  the  size  of  nuclei  may  be  changed  by  somatic  condi- 
tions, by  food  supply,  so  that  in  every  generalization  reached  by  the 
study  of  the  size  of  nuclei,  we  must  be  very  circumspect,  and  not  fancy 
too  easily  that  we  have  reached  a  safe  conclusion  unless  we  have  taken 
into  consideration  all  the  possible  factors  by  which  the  size  may  have 
been  varied. 

In  what  I  have  said  to  you  hitherto  in  regard  to  the  power  of 
growth,  I  have  directed  your  attention  chiefly  to  the  power  of  growth 
as  it  exists  in  a  cell  in  consequence  of  that  cell's  condition.  When  the 
cell  is  in  the  young  state,  it  can  grow  rapidly ;  it  can  multiply  freely ; 
when  it  is  in  the  old  state  it  loses  those  capacities,  and  its  growth  and 
multiplication  are  correspondingly  impeded,  and  if  the  organization  is 
carried  to  an  extreme,  the  growth  and  the  multiplication  of  the  cell 
cease  altogether. 

We  find,  however,  that  there  is  something  a  little  more  complicated 
yet  to  be  considered,  for  it  is  not  merely  a  question  of  the  capacity  of 
the  cells,  but  also  of  the  exercise  of  that  capacity,  which  we  must  deal 
with.  Here  comes  in  a  factor  which  we  learn  from  the  study  of  regen- 
eration. The  phenomena  of  regeneration  are  very  important  and  very 
instructive.  We  shall  come  to  those  in  a  moment.  It  will  make  our 
study  of  regeneration  clearer,  more  significant,  I  think,  if  we  pause  for 
a  moment  to  consider  certain  fluctuations  in  the  natural  development 
of  the  organism.  We  see,  for  instance,  in  the  brain  that  early  the  cells 
begin  to  assume  the  character  of  nerve  cells  and  that  thereafter  their 
multiplication  ceases.  But,  curiously,  there  will  be  a  spot  in  the  spinal 
cord,  for  example,  where  the  change  of  the  cells  into  nerve  cells  has  not 
taken  place,  and  from  that  growth  will  go  on.  Cells  will  migrate  from 
that  spot  and  reach  their  ultimate  destination.  When  the  child  is 
born  it  is  very  incapable  of  movement.  There  is  scarcely  more  tlian 
the  power  of  twitching  about  in  a  disorderly  fashion.  Its  muscles  can 
contract,  to  be  sure,  but  any  sort  of  motion  that  implies  a  harmonious 
working  together  of  various  muscles,  the  baby  at  birth  is  quite  incapable 
of.  This  phenomenon  is  doubtless  due  to  the  fact  that  the  cerebellum, 
the  small  brain,  is  as  yet  imperfectly  developed.  If  we  examine  the 
brain  of  the  child  at  birth,  we  find  at  the  edge  of  the  cerebellum  a  line 


453  AGE,    GEOWTH   AND    DEATH 

along  which  the  loroclnction  of  new  cells  is  going  on.  These  new  cells 
migrate  over  the  surface  of  the  cerebellum  without  changing  at  all  into 
nerve  cells.  They  form  a  distinct  layer  which  is  well  known  to  every 
investigator  of  brain  structure,  and  presently  after  birth  these  cells 
accomjolish  a  second  migration,  but  in  a  different  direction.  Instead 
of  moving  in  a  constant  current  over  the  surface  of  the  brain,  each 
one  takes  a  vertical  pathway  from  the  surface  down  towards  the  in- 
terior of  the  cerebellum;  and  arrived  there  it  changes  and  becomes  a 
nerve  cell,  or  at  least  a  part  of  them  do ;  and  with  that  the  machinery  of 
the  cerebellum  is  complete.  Thus,  structurally,  the  cerebellum  at 
birth  is  an  uncompleted  organ.  Now,  the  cerebellum  is  that  portion 
of  the  brain  which  regulates  the  combination  of  muscular  movements, 
which  secures  that  which  the  physiologists  term  coordination  of  move- 
ments, and.it  is  not  until  the  cerebellum  has  been  perfected  that  it  can 
perform  this  function.  Were  there  not  some  provision  of  this  special 
sort  for  allowing  cells  to  be  produced  and  added  to  the  brain,  the  full 
complexity  of  the  brain  could  not  be  attained,  because  after  the  cells 
have  begun  to  change  into  nerve  cells  they  lose  their  power  of  multipli- 
cation, and  this  is  a  device  very  exquisite  in  its  working  to  supply  to  the 
brain  the  requisite  number  of  cells  to  give  it  its  full  measure  of  com- 
plexity. 

Another  instance  of  the  reservation  of  cells  of  a  simple  type  is 
afforded  us  by  the  skin,  about  which  I  shall  have  something  more  to 
say  in  a  few  moments  when  we  speak  of  the  process  of  regeneration. 
It  is  not  only  in  the  period  of  childhood,  and  not  only  in  the  cerebellum, 
that  we  find  cells  exist  such  as  I  have  just  described  to  you,  but  it  is 
in  other  parts  of  the  body  also  and  at  other  periods  of  life  that  we  find 
the  like  phenomena ;  and  in  part  I  have  already  referred  to  these.  You 
remember  I  told  you  in  a  previous  lecture  there  is  always  in  the  body, 
even  at  the  extreme  of  life,  a  store  of  cells  of  the  young  type,  which  is 
garnered  in  the  marrow  of  the  bones.  The  cells  in  question  can  mul- 
tiply, and  their  descendants  in  part  undergo  a  change  in  consequence 
of  which  they  are  converted  into  blood  corpuscles.  The  undifferen- 
tiated or  young  cells  are  preserved  in  the  marrow  precisely  for  the  pur- 
pose of  making  up  the  necessary  number  of  blood  corpuscles  to  replace 
those  which  are  lost  either  by  accident  or  in  consequence  of  normal 
physiological  ^Drocesses.  I  mentioned  to  you  that  in  the  lining  of  the 
intestine  there  is  a  constant  loss  of  cells  and  we  find  in  every  simple 
gland  of  the  intestine,  in  every  little  gland  of  the  stomach,  a  center  for 
cell  production,  a  center  where  there  is  a  group  of  cells  which  are  not 
differentiated,  but  retain  their  simj)le  organization. 

I  could  multiply  these  instances  almost  indefinitely,  but  perhaps  it 
will  be  better  to  call  your  attention  to  an  illustration  of  quite  a  different 
sort.  We  know  that  in  order  to  have  a  very  complex  organization,  the 
number  of  cells  in  the  body  must  be  very  large  indeed.      Obviously  a 


POPULAR    SCIENCE   MONTHLY  464 

small  insect,  a  mosquito  or  a  little  beetle,  whatever  it  may  be,  is  not 
big  enough  to  have  a  great  many  cells ;  and,  unless  it  has  a  great  many, 
it  can  not  attain  the  differentiation  of  complicated  organs  such  as  we 
possess.  Now,  the  lower  animals  are  born,  so  to  speak,  earlv,  and  as 
soon  as  they  hatch  out,  they  have  to  support  themselves.  We  see  that, 
for  instance,  in  caterpillars.  They  are  born  very  little  creatures,  but 
each  caterpillar  must  look  out  for  itself,  obtain  its  own  food,  move 
about  to  that  food,  must,  when  the  food  is  swallowed,  digest  it,  and 
must  carry  on  the  correlated  functions  of  secretion  and  excretion;  it 
must  breathe.  In  order  to  do  all  this  the  larva,  or  young  caterpillar, 
to  follow  our  special  instance,  must  have  some  differentiation  already 
established;  but,  as  we  have  already  learned,  differentiation  impedes 
growth.  In  other  words,  in  such  a  larva  the  multiplication  of  cells  is 
held  back  by  the  very  demands  of  the  conditions  of  its  existence.  If  it 
is  to  have  organs  which  are  to  function,  it  must  have  differentiated 
parts,  and,  if  it  is  differentiated,  its  growth  power  must  be  sacrificed. 

Now  how  has  nature  proceeded  in  order  to  produce  a  higher  type  of 
animal,  one  in  which  the  number  of  cells  is  much  greater?  A'^ery  in- 
geniousl3\  She  provides  the  developing  organism  with  a  food  supply 
which  it  carries  itself.  If,  for  instance,  you  recall  the  egg  of  the 
salamander,  which  I  showed  you  iipon  the  screen,  you  will  rememljcr 
that  that  is  a  structure  of  considerable  size,  and  its  size  is  due  to 
the  accumulation  of  food  material,  material  which  we  designate 
by  the  term  yolk  granules,  which  lie  in  the  living  protoplasm  of  that 
germ.  This  supply  of  food  is  so  great  that  it  will  last  the  organ- 
ism a  considerable  period.  While  it  is  growing  it  has  nothing  to  do 
but  to  digest  that  food  supply  which  it  already  possesses.  It  does  not 
have  to  exert  itself  to  obtain  it,  and  no  further  digestive  process  is 
necessary  than  that  inherent  in  all  living  protoplasm.  So  the  3'oung 
salamanders  develop  in  a  most  advantageous  condition,  and  can  actu- 
ally produce  a  much  greater  number  of  cells  because  it  is  possible,  with 
this  internal  food  supply,  for  the  growth  to  go  on  only  with  the  cells 
of  the  embryonic  or  youthful  type  for  a  considerable  period,  and  then, 
when  their  number  has  consideral)ly  increased,  steps  in  the  process  of 
differentiation. 

In  the  higher  animals  this  accumulation  of  food  for  the  nourish- 
ment of  the  germ  is  carried  yet  further.  As  you  know,  the  egg  of  the 
bird  is  much  bigger  than  that  of  the  salamander,  and  in  the  highest 
animals,  in  the  mammals,  tliere  are  other  si)ecial  contrivances  which 
nature  has  introduced  to  secure  ample  and  aderpiate  nourishment  of 
the  developing  germ.  There  the  perfection  of  the  jirocess  is  made  yet 
greater  and  in  these  forms  Hiere  is  a  long  |)eriod  during  wbicli  Ihc  pr'o- 
duction  of  cells  goes  on  ;  the  cells  all  remain  simple,  and  l)y  the  time 
they  begin  to  change  the  number  of  cells  is  so  great  that  tlie  possibili- 
ties of  an  infinite  variety,  almost,  oF  pcfulinrit  ii'^  in  Ibciii  are  given,  and 


465 


AGE,    GROWTH   AND    DEATH 


there  are  cells  enough  to  allow  this  variety  to  be  worked  out.  This  we 
call  the  embryonic  type  of  develojDment. 

We  see,  therefore,  that  nature  has  recognized  a  restriction  which 
she  herself  has  put  upon  development,  the  restriction  which  obliges 
development,  if  it  is  to  be  ample,  to  prolong  the  accumulation  of  the 
undifferentiated  cells.  In  response  to  that  condition,  she  substitutes 
for  higher  types  of  animal  that  development  which  we  call  embryonic, 
leaving  for  the  lower  type  that  which  we  call  larval.  Thus  we  see  in 
the  growth  and  formation  of  the  higher  animals  and  in  the  history  of 
the  comparative  development  of  the  animal  kingdom,  fresh  illustra- 
tions of  the  great  importance  of  the  3'oung  type  of  cells. 

We  can  see  the  same  thing  also  in  regard  to  regeneration.  The 
regenerative  process  depends  to  a  large  extent  upon  partial  differentia- 
tion, or  even  upon  its  total  absence.  Eegeneration  is  a  most  interest- 
ing and  wonderful  process.  I  took  pains  only  this  afternoon  to  look 
at  that  famous  classic  by  tlie  Dutch  Abbe  Trembley  on  hydroids  or 
polyps  as  he  calls  them.  "  The  Fresh  Water  Polyps,"  a  book  published 
in  1744,  was  well  printed,  and  on  such  good  paper  that  it  looks  to-day 


Fig.  55. 


Vignette  from  Tkeivibley's  Classic  Memoir,  representing-  Trembley  making 
his  experiments  on  regenerition  in  freshi-vvater  polyps. 


almost  like  a  new  book.  He  made  the  curious  experiment  of  cutting 
one  of  these  minute  fresh  water  polyps — they  are  perhaps  an  eighth 
of  an  inch  long — in  two,  and  made  the  startling  discovery  that  each 
half  of  the  polyp  would  make  up  what  the  other  half  had  deprived  it 
of:  each  half,  in  other  words,  would  become  a  new  polyp.  That  which 
was  lost  was  regenerated.  After  him  came  a  series  of  yet  more  remark- 
able experiments  by  the  famous  Italian  naturalist,  Spallanzani,  one  of 
the  masters  of  experimental  research,  and  he  discovered  that  regenera- 
tion was  a  property  which  was  not  peculiar  by  any  means  to  polyps, 

VOL.  LXXI. — 80 


POPULAR    SCIENCE   MONTHLY 


466 


but  existed  in  the  earth-worms,  and  even  among  vertebrates;  for  he  it 
was  who  discovered  that  if  the  head  of  an  earthworm  be  cut  off,  the 
worm  will  form  a  new  head  with  a  new  brain  and  a  new  mouth.  He 
first  discovered  that  if  you  cut  off  the  tail  of  a  salamander  a  new  tail 
will  grow  out.  He  it  was,  moreover,  who  discovered  that  this  power 
of  replacing  the  lost  part  is  greater  in  the  young — greater  in  the  earlier 
stage  than  in  the  later.  This  indicates  in  a  general  way  the  nature 
and  process  of  regeneration.  We  have  many  kinds  of  regeneration; 
we  may  have  that  of  the  single  cell  or  that  of  the  whole  organism. 

We  pass  now  to  the  next  of  our  slides,  which  represents  a  creature 
of  the  kind  called  Stentor.     It  is  a  single  cell.     Here  is  the  nucleus  of 


B 


n 


Fig.  oG.    iSleiUor. 


the  cell ;  its  protoplasmic  body  is  large,  and  something  of  the  structure 
of  this  I  have  told  you  in  a  previous  lecture.  A  German  investigator, 
Professor  Gruber,  has  succeeded  in  dividing  one  of  these  Stentors,  a 
unicellular  creature,  animalcule,  common  in  fresh  water,  into  three 
parts  in  such  a  method  of  cutting  as  is  illustrated  by  the  figure  on  the 
left.  Each  of  the  three  parts  will  then  restore  itself  and  become  a 
complete  Stenior.  In  such  experiments  the  protoplasm  over  the  nu- 
cleus begins  to  grow;  gradually  the  original  form  is  again  assumed; 
the  creature  grows  larger  and  larger,  until  each  piece  acquires  the 
parent  size,  and,  so  far  as  we  can  see  with  the  ordinary  microscopic 
examination,  identically  the  parental  structure.      'J'hat  which  was  lost 


467 


AGE,    GROWTH    AND    DEATH 


has  been  regenerated.      We  learn,  then,  that  regeneration  is  a  faculty 
which  a  single  cell,  a  single  unit,  may  possess. 

Our  next  picture  demonstrates  a  similar  phenomenon.  These  are 
muscle  fibers  which  have  been  injured.  Now  every  muscle  fiber  con- 
tains in  its  interior  its  contractile  substance,  in  regard  to  which  I 
have  already  spoken  to  you,  but  it  also  contains  a  certain  amount  of 
substance  which  is   still   undifferentiated   protoplasm.     Now   when   a 


Fig.  57.    Striated  Muscle  Fibers  in  Process  of  Regeneration. 

muscle  fiber  of  this  sort  is  injured,  we  find  that  the  muscular  structure, 
properly  so  called,  will  in  many  cases  quite  disappear,  but  then  the 
protoplasmic  material,  which  is  the  undifferentiated  substance,  will 
begin  to  grow,  and  the  nuclei  will  begin  to  multiply.  This  may  happen 
at  the  end  of  a  muscle  fiber,  producing  there  a  considerable  mass  of 
protoplasm,  with  nuclei  multiplying  in  it,  or  we  may  find  a  chain  of 
nuclei,  each  with  its  separate  court  of  protoplasm  body;  such  nuclei 
will  multiply.  When  the  increase  of  the  undifferentiated  protoplasm 
has  gone  on  far  enough,  the  injured  muscle  will  produce  again  the 
muscular  substance  proper — the  contractile  fibrils.  Muscular  fiber,  in 
other  words,  can  be  regenerated  by  itself. 

A  similar  thing  to  this  happens  when  a  nerve  is  cut  across.  The 
nerve  fiber,  which  is  connected  with  the  nerve  cell  from  which  it  arose, 
is  capable  of  growing  out  again.  It  can  regenerate  itself,  and  that 
is  a  well-known  phenomenon,  and  in  many  surgical  cases  it  becomes  a 
phenomenon  of  very  great  importance. 

Let  us  go  back  to  another  familiar  figure.  Here  is  represented  the 
lining  layer  of  the  resophagus  with  the  cells  composing  it,  the  upper 
anes  being  horny,  the  lower  ones  those  which  are  capable  of  active 


POPULAR    SCIENCE   MONTHLY  468 

growth.  We  are  rather  dull.  We  do  not  often  stop  to  think  about 
things.  We  buy  a  new  horse  which  comes  from  the  country,  has  never 
seen  a  train;  drive  him  to  the  station,  and  are  frightened,  perhaps, 
because  the  horse  himself  is  so  much  alarmed — possibly  have  a  narrow 
escape  because  of  the  excitement  which  his  first  sight  of  a  train  causes 
him.  But  that  horse,  after  a  few  months'  discipline,  will  scarcely  turn 
his  ear,  much  less  his  head,  to  look  at  the  train  which  a  short  time 
before  so  frightened  him  and  held  his  attention  that  nothing  else  could 
get  into  his  mind  save  the  fright  that  train  gave  him.  So  we,  too, 
act  a  good  deal  like  the  horse.  We  see  a  thing  the  first  time  and  it 
surprises  us;  the  next  time  it  seems  like  an  old  acquaintance,  a  thing 


Fig.  58.    Section-  of  the  Epithelial  Lining  of  the  HUxMan  CE-^ophagus. 

familiar  and  therefore  unregarded.  I  say  this  apropos  of  the  skin. 
How  many  of  you  have  thought  what  the  lesson  of  the  skin  is  in 
regard  to  the  power  of  growth?  Spring  is  coming;  we  shall  soon  be 
taking  to  our  boats,  rowing  or  canoeing,  and  the  first  day  we  do  so 
doubtless  we  shall  have  blisters  upon  our  hands,  and  the  outer  part  of 
the  skin,  raised  by  the  blister,  will  probably  fall  off  and  be  lost  alto- 
gether. The  softer,  underlying  skin  will  be  exposed,  will  be  sensitive 
and  uncomfortable  for  a  while,  but  soon  the  cells  behind  the  surface 
will  assume  a  horny  character,  the  cells  underneath  will  grow  and 
multiply,  and  presently  the  wound  will  be  healed  over.  Did  you  ever 
stop  to  think  that  that  means  that  there  is  a  reserve  power  of  growth 
in  the  skin  all  the  time?  always  ready  to  act,  to  come  forward,  waiting 


469  AGE,    GROWTH   AND   DEATH 

only  for  the  chance,  and  that  there  is  besides  something  which  keeps 
it  in,  which  holds  it  back,  which  stops  it?  We  call  this  stopping 
physiological  function — inhibition;  we  say  that  the  growth  of  the  skin 
is  inhibited;  though  in  the  deep  part  of  the  skin  all  the  time  there 
are  the  cells  ready  to  grow  as  soon  as  that  power  of  inhibition  is  taken 
away;  while  it  is  active  they  will  not  grow.  The  simple  blister  tells 
us  all  that.  There  is,  then,  a  power  of  regulation  which  expresses 
itself  in  this  inhibitory  effect.  When  a  salamander  has  its  tail  cut 
off  by  the  experimenter  and  the  new  tail  grows,  just  enough  is  pro- 
duced. The  new  tail  is  like  the  old.  The  tissues  grow  out  until  the 
volume  of  that  which  is  lost  is  replaced,  and  then  they  stop.  But  if 
the  tail  should  be  cut  off  again,  regeneration  would  occur  again.  The 
experiments  may  be  repeated  many  times  over.  It  indicates  to  us  that 
always  the  growing  power  is  there,  but  it  is  held  in  check.  What  that 
check  may  be  is  one  of  the  great  discoveries  we  are  now  longing  for. 
The  discovery,  when  made,  is  likely  to  prove  of  great  practical  im- 
portance. The  phenomenon  of  things  escaping  from  inhibitory  control 
and  overgrowing,  is  familiar.  Such  escapes  we  encounter  in  tumors, 
cancers,  sarcoma  and  various  other  abnormal  forms  of  growth  that 
occur  in  the  body.  They  are  due  to  the  inherent  growth  power  of 
cells  kept  more  or  less  in  the  young  type,  which  for  some  reason  have 
got  beyond  -the  control  of  the  inhibitory  force,  the  regulatory  power 
which  ordinarily  keeps  them  in.  Ko  picture  of  the  growth  or  develop- 
ment of  the  living  animal  would  be  complete  if  it  confined  its  attention 
•only  to  the  power  of  growth  in  relation  to  cytomorphosis.  It  must 
also  include  the  contemplation  and  study  of  this  regulatory  power  of 
the  organs.  Experiments  are  being  made  in  many  places,  minds  are 
at  work  in  many  laboratories  upon  this  problem  of  regulation  of 
structure  and  growth.  Much  is  to  be  hoped  from  such  researches; 
not  merely  insight  into  the  normal  development,  but  insight  also  into 
the  abnormal.  Nothing,  perhaps,  is  more  to  be  desired  at  the  present 
time  than  that  we  should  gain  scientific  insight  into  the  regulatory 
power  which  presides  over  growth.  It  would  be  of  immense  medical 
importance.  Could  we  understand  it,  and  could  we  from  our  under- 
standing derive  some  practical  application  of  our  scientific  discoveries 
in  this  field,  we  could  say  of  it  justly  that  it  was  as  noteworthy  a  con- 
tribution to  medical  knowledge  as  the  discovery  of  the  germs  of  disease, 
and  would  doubtless  prove  equally  beneficial  to  mankind.  Although, 
then,  the  study  which  I  have  been  laying  before  you  must  necessarily 
seem  in  many  respects  abstruse  and  far  away  from  practical  applica- 
tions, we  learn  that  it  is  not  really  so,  and  that  it  leads  by  no  very 
remote  path  to  the  consideration  of  problems  the  useful  applications  of 
which  are  immediately  obvious  to  every  one. 

We  find  in  the  process  of  regeneration  that  it  is  always  the  young 
cell  which  plays  the  principal  part.     This  is  beautifully  illustrated  in 


POPULAR    SCIENCE   MONTHLY 


470 


the  picture  upon  the  screen.  There  is  a  little  creature,  which  many 
of  you  have  seen  in  the  garden,  consisting  of  joints,  which  rolls  itself 
up  into  a  little  hall,  and  therefore  is  often  called  the  "  pill-hug."  It 
is  not,  however,  an  insect  or  a  hug,  properly  so-called,  but  belongs 
to  a  family  of  crustaceans.  It  has  on  its  head  a  little  feeler  which 
we  call  the  antenna.     The  particular  kind  of  arthropod,  the  antenna 


„if^r^.  * , 


Fig.  59.     Sections  through  the  Antenna  of  Oniseus  in  Various  Stages  of  Regenera- 
tion AFTER  Amputation.    After  Ost. 

of  which  has  been  studied  and  drawings  of  it  made  to  furnish  us  this 
plate,  is  known  by  the  name  of  Oniseus.  In  his  researches  the  experi- 
menter. Dr.  Ost,  cut  oil:  the  antenna  in  the  middle  of  a  joint  and 
found  that  it  rapidly  healed  over.  Here  is  pictured  the  process  of 
healing  going  on.  Part  of  the  antenna  has  been  cut  off  in  this  case, 
the  wound  was  healed  over  here,  No.  1,  a,  the  new  tissue  has  begun  to 
grow.  No.  2,  h,  and  the  cells  at  this  point  are  very  simple  in  character. 
They  spread  out  and  grow,  and  then,  within  the  interior  of  the  hard 
shell  of  the  feeler,  a  retraction  of  the  substance  occurs,  and  the  new 
growing  cells  within  this  space  gradually  begin  to  shape  themselves  cut. 


471 


AGE,    GROWTH   AND    DEATH 


No.  3,  l,  and  we  see  presently  an  accumulation  of  cells  which  is 
assuming  a  definite  form.  No.  4,  l,  that  in  the  next  figure  has  clearly- 
become  the  promise  or  beginning  of  a  new  terminal  joint,  Fig.  60. 
The  minute  study  of  this  process  has  shown  that  the  regeneration 
depends  practically  exclusively  upon  the  cells  of  the  young  type,  and 
that  after  they  have  grown  out  and  accumulated  here  in  this  manner, 
No.  3,  l,  some  of  them  undergo  differentiation,  becoming  muscle  cells; 
others  change  in  the  manner  indicated  here, 
where  we  see  a  commencing  alteration  of 
the  nuclei,  which  is  further  accented  in 
Fig.  60,  and  leads  to  such  a  grouping  of  the 
cells  that  the  glands,  which  were  originally 
present  there,  are  also  reproduced.  The 
regenerative  process,  theii,  clearly  illustrates 
to  us  from  another  point  of  view  the  great 
importance  of  the  young  type  of  cells. 

This  completes  the  evidence  which  my 
time  permits  me  to  lay  before  you  in  order 
to  convince  you  that  really  the  young  type 
of  cells  is  physiologically  and  functionally 
important,  that  it  really  does  possess  the 
power  of  growth  such  as  I  have  attributed 
to  it. 

We  will  pass  now  to  another  part  of  our 
subject,  with  which  the  lecture  will  close. 
Age  represents  the  result  of  a  progressive 
cytomorphosis.  We  have  learned  that  of 
cytomorphosis  death  is  the  end,  the  culmina- 
tion. It  is  a  necessary  result  of  the  modifi- 
cation and  change  of  structure  which  goes 
on  in  every  individual  of  our  species  and  of 
all  the  higher  animals.  We  are  familiar 
with  the  death  of  cells.  It  occurs  constant- 
ly and,  as  I  have  endeavored  to  explain  to 

you,   it  plays  a  great  part  in  life.       It  pro-     «-  cicatricial  tissue ;  b,  regenerated 

tissue  :  j,  new  joint ;  cu,  cutleula 

motes  the  periormance  of  various  functions    (oid  shell).    Magnified, 
which  are  of  advantage  to  the  body  as  r 

whole,  which  could  not  be  accomplished  without  the  death  of  some  cells. 
But  the  death  which  we  have  in  mind  when  we  speak  ordinarily  of  death 
is  something  different  from  this.  It  is  the  death  of  the  whole.  But 
even  the  death  of  the  whole  has  its  strange  complications.  A  great 
deal  of  our  knowledge  of  the  functioning  of  the  body  is  due  to  the 
fact  that  the  parts  do  not  die  when,  as  we  commonly  say,  the  body  as 
a  whole,  the  individual,  is  dead.     The  organ  is  alive  and  well.     One 


Fig.  60.  Section  through  a  Re- 
generating Antenna  of  Onis- 
cus.  After  Ost.  Advanced  stage,  in 
which  the  young  new  joint  is  al- 
ready shaped  within  the  old  shell. 


POPULAR    SCIENCE    MOXTHLY  472 

of  the  most  impressive  sights  which  I  liave  ever  seen  has  been  the 
sight  of  the  heart  of  a  quadruped,  a  dog,  continuing  to  beat  after  it 
had  been  taken  out  from  the  body.  The  dog  was  dead — the  rest  of 
his  body  was  dead — but  the  heart  lay  upon  the  physiologist's  table 
beating.  The  experimenter  could  supply  it  with  the  necessary  circula- 
tion. He  could  give  stimuli  to  it,  and  under  these  favorable  condi- 
tions make  important  discoveries  in  regard  to  the  functioning  of  the 
heart.  So  too  I  myself  made  experiments  upon  a  muscle  once  part  of 
a  living  dog,  separated  entirely  from  the  parent  body,  supplied  with 
its  own  artificial  circulation,  and  from  those  experiments  was  able  to 
discover  some  new  unexpected  results  in  regard  to  the  nutrition  of 
the  muscle,  and  the  changes  which  chemically  go  on  in  it.  This  over- 
living, then,  of  the  parts  of  the  body,  their  separate  life,  is  something 
which  we  must  familiarize  ourselves  with,  and  the  great  importance  of 
which  we  must  carefully  acknowledge,  for  much  of  the  benefit  which 
the  medical  practitioner  is  able  to  render  to  us  and  to  our  friends 
to-day  is  due  to  the  knowledge  which  has  been  derived  experimentally 
from  the  study  of  the  over-living  or  surviving  parts  of  a  body  which  as 
a  whole  was  dead. 

Death  is  not  a  universal  accompaniment  of  life.  In  the  lower 
organisms  death  does  not  occur  as  a  natural  and  necessary  result  of 
life.  Death  with  them  is  purely  the  result  of  an  accident,  some  ex- 
ternal cause.  "Natural  death  is  a  thing  which  has  been  acquired  in 
the  process  of  evolution.  Why  should  it  have  been  acquired?  You 
will,  I  think,  readily  answer  this  question  if  you  hold  that  the  views 
which  I  have  been  bringing  before  you  have  been  well  defended,  by 
saying  that  it  is  due  to  differentiation,  that  when  the  cells  acquire  the 
additional  faculty  of  passing  beyond  the  simple  stage  to  the  more  com- 
plicated organization,  they  lose  something  of  their  vitality,  something 
of  their  power  of  growth,  something  of  their  possibilities  of  perpetua- 
tion ;  and  as  the  organization  in  the  process  of  evolution  becomes  higher 
and  higher,  this  necessity  for  change  becomes  more  and  more  im- 
perative. But  it  involves  the  end.  Differentiation  leads  up,  as  its 
inevitable  conclusion,  to  death.  Death  is  the  price  we  are  obliged  to 
pay  for  our  organization,  for  the  differentiation  which  exists  in  us.  Is 
it  too  high  a  price?  To  that  organization  we  are  indebted  for  the 
great  array  of  faculties  with  which  we  are  endowed.  To  it  we  are 
indebted  for  the  means  of  appreciating  the  sort  of  world,  the  kind  of 
universe,  in  which  we  are  placed.  To  it  we  are  indebted  for  all  the 
conveniences  of  existence,  by  which  we  are  able  to  carry  on  our  physio- 
logical processes  in  a  far  better  and  more  comfortable  manner  than  can 
the  lower  forms  of  life.  To  it  we  are  indebted  for  the  possibility  of 
those  human  relations  which  are  among  the  most  precious  parts  of  our 
experience.  And  we  are  indebted  to  it  also  for  the  possibility  of  the 
higher  spiritual   emotions.     .Ml   this  is  what  we  have  bought  at  the 


473  AGE,    GROWTH   AND    DEATH 

price  of  death,  and  it  does  not  seem  to  me  too  much  for  us  to  pay. 
We  would  not,  I  think,  any  of  us,  wish  to  go  back  to  the  condition  of 
the  lowly  organism  which  might  perpetuate  its  own  kind  and  suffer 
death  only  as  a  result  of  accident  in  order  that  we  might  live  on  this 
earth  perpetually;  we  would  not  think  of  it  for  a  moment.  We  accept 
the  price.  Death  of  the  whole  comes,  as  we  now  know,  whenever  some 
essential  part  of  the  body  gives  way — sometimes  one,  sometimes 
another ;  perhaps  the  brain,  perhaps  the  heart,  perhaps  one  of  the  other 
internal  organs  may  be  the  first  in  which  the  change  of  cytomorphosis 
goes  so  far  that  it  can  no  longer  perform  its  share  of  work,  and  failing, 
brings  about  the  failure  of  the  whole.  This  is  the  scientific  view  of 
death.  It  leaves  death  with  all  its  mystery,  with  all  its  sacredness; 
we  are  not  in  the  least  able  at  the  present  time  to  say  what  life  is,  still 
less,  perhaps,  what  death  is.  We  say  of  certain  things — ^they  are  alive ; 
of  certain  others — they  are  dead ;  but  what  the  difference  may  be,  what 
is  essential  to  those  two  states,  science  is  utterly  unable  to  tell  us  at 
the  present  time.  It  is  a  phenomenon  with  which  we  are  so  familiar 
that  perhaps  we  do  not  think  enough  about  it. 

In  the  next  lecture  there  will  be  some  other  general  aspects  of  our 
subject  to  present  to  you,  and  a  formulation  of  the  general  conclusions 
towards  which  all  the  lectures  have  aimed. 


AGE,    GRO]yTH   AND   DEATH  509 


THE   PEOBLEM   OF  AGE,   GROWTH  AXD   DEATH 

By  CHARLES  SEDGWICK  MINOT,  LL.D.,  D.Sc. 
JAMES  STIIJ.MAN   PROFESSOR  OF  CO>;PARATIVE  ANATOMY,   HARVARD  MEDICAL  SCHOOI, 

VI.     The  Four  Laws  of  Age 

Ladies  and  Gentlemen:  I  have  referred  in  these  lectures  repeatedly 
to  the  cell  and  its  two  component  parts,  the  nucleus  and  the  proto- 
plasm. To-night  I  shall  have  only  a  few  references  to  make  directly 
to  these,  and  shall  pass  on  for  the  latter  part  of  the  hour  to  another 
class  of  considerations  bearing  upon  the  problem  of  age.  Before  we 
turn  to  these  new  considerations,  however,  I  wish  to  say  a  few  words 
by  way  of  recapitulation  concerning  the  changes  in  the  cells  as  corre- 
sponding to  age.  Cells,  as  you  know  from  what  I  have  told  you, 
undergo  in  the  body  for  the  greater  part  a  progressive  change  which 
we  call  their  differentiation.  We  may  say  that  there  are  four  kinds 
of  cells  for  purposes  of  an  elementary  classification  to  be  used  in  a 
simple  exposition  like  the  present.  The  first  kind  are  those  cells  of 
the  young  type,  in  which  the  protoplasm  is  simple,  and  shows  as  yet 
no  trace  of  differentiation.  These  cells  are  capable  of  rapid  multipli- 
cation, and  some  of  them  are  found  still  persisting  in  various  parts 
of  the  adult  body,  and  serve  to  maintain  the  growth  of  the  body  in  its 
mature  stage.  Another  class  of  cells  presents  to  us  the  curious  spectacle 
of  a  partial  differentiation;  such  are  the  muscle  fibers  by  which  we 
accomplish  our  voluntary  movements.  These  fibers  consisted  originally 
only  of  protoplasm  with  the  appropriate  nuclei,  but,  as  they  are  differen- 
tiated, part  of  the  protoplasm  changes  into  contractile  substance. 
Another  part  remains  pure  protoplasm  unaltered.  If  now  the  mus- 
cular or  contractile  portion  of  the  fiber  be  destroyed,  the  undifferen- 
tiated part  of  the  protoplasm  then  shows  that  it  has  still  the  power  of 
growth.  It  has  only  been  held  back  by  the  condition  of  organization, 
and  we  see  in  the  regeneration  of  these  fibers  evidence  of  the  fact  that 
so  long  as  the  protoplasm  is  undifferentiated  it  has  the  power  of 
growth,  which,  however,  does  not  reveal  itself  unless  an  opportunity  is 
afforded.  Third,  we  come  to  the  cells  which  are  moderately  differen- 
tiated; such,  for  instance,  are  the  cells  of  the  liver,  and,  if  for  any 
reason  a  portion  of  the  liver  be  injured  by  accident  or  disease,  we  find 
that  these  partially  differentiated  cells  reveal  at  once  that  they  have  a 
limited  power  of  growth  still  left.  If  we  pass  on  to  the  fourth  class, 
that  in  which  differentiation  is  carried  to  the  highest  extreme,  we  find 
that  the  cells  do  not  have  the  power  of  multiplication.      Such  aref  the 


5IO 


POPULAR   SCIENCE  MONTHLY 


^ 


nerve  cells  by  which  the  higher  functions  of  the  body  are  carried  on. 
They  represent  the  extreme  of  cellular  differentiation,  and  almost  never 
do  we  see  these  cells  multiplying  after  the  differentiation  is  accom- 
plished. Presented  in  this  form,  we  then  recognize,  it  seems  to  me 
clearly,  the  effect  of  differentiation  upon  the  growth  of  cells.  The  facts 
are  clear  as  to  their  meaning. 

We  can,  however,  proceed  a  little  farther  than  this,  because  we  can 
actually  determine,  approximately  at  least,  the  rate  at  which  cells  mul- 
tiply, and  that  we  can  do  by  means  of  determining  the  mitotic  index. 
The  mitotic  index  is  the  number  of  cells  to  be  found  at  any  given 
moment  in  the  active  jDrocess  of  division  out  of  a  total  of  one  thousand 
cells. 

May  I  pause  a  moment  to  recall  this  picture  to  you  and  ask  you  to 
notice  at  this  point  the  curious  darker  spot  which  represents  a  nucleus 
in  process  of  division  ?     You  will  see  it  would 
_^^  be  easy  in  such  a  preparation  as  this  to  count 

the  nuclei  one  by  one  until  one  had  got  up  to  a 
thousand,  and  to  record,  as  one  went  along, 
how  many  of  the  nuclei  are  in  process  of  divi- 
sion, for  the  nucleus  in  division  is  easily  recog- 
nized. This  process  of  division  is  named 
mitosis :  the  figure  which  the  nucleus  presents 
while  it  is  undergoing  division  we  call  a  mi- 
totic figure.  Counting  the  dividing  nuclei,  we 
may  determine  that  in  a  thousand  cells  there 
are  a  given  number  which  have  nuclei  in  proc- 
ess of  division,  and  such  a  number  I  propose  to 
call  "  the  mitotic  index."  I  wish  now  only  to 
call  to  you  attention  this  picture  because  it 
enables  me  to  illustrate  before  5^ou  the  method 
of  measuring  the  mitotic  index. 

In  the  rabbit  embryo  at  seven  and  one  half 
days,  I  have  found  by  actual  count  that  there 
are  in  the  outer  layer  of  cells,  known  techni- 
cally as  the  ectoderm,  18  of  these  divisions  per 
thousand.  In  the  middle  layer,  technically  the 
mesoderm,  17,  and  in  the  inner  layer,  the  ento- 
derm, 18.  At  ten  days  we  find  the  number  al- 
ready reduced,  and  the  figures  are,  respectively, 
14,  13  and  15,  and  for  the  cells  of  the  blood 
only  10.  There  has  already  been  a  great  reduction.  In  the  next  phase 
of  development  (rabbit  embryo  of  thirteen  daysj,  we  find,  however, 
that  the  parts  are  growing  irregularly,  some  faster,  some  slower.  We 
note  that  wherever  a  trace  of  differentiation  has  occurred,  the  rate  of 
growth  is  diminished :  where  that  differentiation  does,  not  show  itself,  the 


Fig.  61.  Portion  of  the 
Outer  Wall  of  a  Primitive 
Muscular  Segment  of  a  Cat 
Embryo  op  4.6  mm.  Harvard 
Embryological  Collection 
Series  398,  section  115.  Ttie 
resting  nuclei  are  oval,  pale 
and  granular.  The  dividing 
or  mitotic  nuclei,  of  which 
there  are  three,  are  dark,  ir- 
regular in  outline  and  show 
the  chromosomes.  In  this 
case  the  dividing  nuclei  all 
lie  near  the  inner  surface  of 
the  wall.  The  picture  illu- 
strates the  ease  with  which 
mitotic  figures  may  be  recog- 
nized. 


AGE,    GROWTH   AND   DEATH  511 

rate  of  growth  may  even  increase  in  order  to  acquire  a  certain  special  de- 
velopment of  a  particular  part.  So  that  instead  of  uniformity  of  values 
for  the  mitotic  index,  we  get  a  great  variety.  But,  nevertheless,  the 
general  decline  can  be  demonstrated  by  the  figures.  In  the  spinal  cord 
the  index  is  11,  in  the  general  connective  tissue  of  the  body  10;  for  the 
cells  of  the  liver  11 ;  in  the  outside  layer  of  the  skin  10;  in  the  excretory 
organ  6 ;  in  the  tissue  which  forms  the  center  of  the  limb  also  6.  There 
has,  then,  been  a  rapid  decline  in  the  rate  of  cell  multiplication  just  in 
this  period  when  differentiation  is  going  on.  This  is,  so  far  as  I  know, 
an  entirely  new  line  of  research.  The  counting  of  a  thousand  cells  is 
not  a  thing  to  be  done  very  rapidly ;  it  must  be  undertaken  with  patience, 
care,  and  requires  time.  It  has  not,  I  regret  to  say,  been  possible  for 
me  yet  to  extend  the  number  of  these  counts  beyond  those  I  have  given 
you,  but  it  is  easy  to  say  that  in  the  yet  more  differentiated  state,  the 
number  of  cells  in  division  is  constantly  lessened,  and  it  is  only  a  ques- 
tion of  counting  to  determine  the  mitotic  index  accurately.  That  there 
is  a  further  diminution  beyond  that  which  the  mitotic  indices  I  have 
demonstrated  to  you  represent  is  perfectly  certain.  I  only  regret  that 
I  am  not  able  to  give  you  exact  numerical  values. 

I  wish  very  much  that  my  time  permitted  me  to  branch  off  into 
certain  topics  intimately  associated  with  the  general  theme  we  have  been 
considering  together  on  these  successive  evenings,  but  we  can  only 
allude  to  a  few  of  these.  The  first  collateral  subject  on  which  I  wish 
to  speak  to  you  briefly  is  that  which  we  call  the  law  of  genetic  restric- 
tion, which  means  that  after  a  cell  has  progressed  and  is  differentiated 
a  certain  distance,  its  fate  is  by  so  much  determined.  It  may  from  that 
pass  on,  turn  in  one  direction  or  another,  always  progressing,  going 
onward  in  its  cytomorphosis ;  but  the  general  direction  has  been  pre- 
scribed, and  the  possibilities  of  that  cell  as  it  progresses  in  its  develop- 
ment become  more  and  more  restricted.  For  instance,  the  cells  which 
are  set  apart  to  form  the  central  nervous  system  after  they  are  so  set 
apart  can  not  form  any  other  kind  of  tissue.  After  the  nervous  system 
is  separated  in  the  progress  of  development  from  the  rest  of  the  body, 
its  cells  may  become  either  nerve  cells  proper  or  supporting  cells 
(neuroglia),  which  latter  never  acquire  the  nervous  character  proper, 
but  serve  to  uphold  and  keep  in  place  the  true  nervous  elements.  They 
represent  the  skeleton  of  the  central  nervous  system.  After  the  cells 
of  the  nervous  system  are  separated  into  these  two  fundamental  classes 
they  can  not  change.  A  cell  forming  a  part  of  the  supporting  frame- 
work of  the  brain  can  not  become  a  nerve  cell ;  and  a  nerve  cell  can  not 
become  a  supporting  cell.  The  destiny  of  them  becomes  more  and 
more  fixed,  their  future  possibilities  more  and  more  limited,  as  their 
cytomorphosis  goes  on. 

The  law  of  genetic  restriction  has  a  very  important  bearing  upon 
questions  of  disease.     When  disease  occurs,  the  cells  of  the  body  offer 


512  POPULAR   SCIENCE  MONTHLY 

to  us  two  kinds  of  spectacles.  Sometimes  we  see  that  the  cells  causing 
the  diseased  condition  are  more  or  less  of  the  sort  which  naturally  be- 
long in  the  body;  that  they  are  present  where  they  do  not  belong,  or 
they  are  present  where  they  ought  to  be,  but  in  excessive  quantity. 
There  is  a  kind  of  tumor  which  we  call  a  bony  tumor.  It  consists  of 
bone  cells  such  as  are  naturally  present  in  the  body,  but  that  which 
makes  this  growth  of  bone  a  tumor  is  its  abnormal  dimensions,  or  per- 
haps its  being  altogether  in  the  wrong  place.  The  second  sort  of 
pathological  alteration,  which  I  had  in  mind,  is  that  in  which  the  cells 
really  change  their  character.  ISTow,  the  young  cells  are  those  which 
can  change  most;  in  which  the  genetic  restriction  has  least  come  into 
play ;  and  accordingly  we  find  that  a  large  number  of  dangerous,  morbid 
growths,  tumors,  arise  from  cells  of  the  young  type,  and  these  cells, 
having  an  extreme  power  of  multiplication,  grow  rapidly,  and  they  may 
assume  a  special  character  of  their  own;  their  genetic  restriction  has 
not  gone  so  far  that  all  their  possibilities  of  change  in  the  way  of  differ- 
entiation have  been  fixed;  there  is  a  certain  range  of  possibilities  still 
open  to  them,  and  they  may  turn  in  one  direction  or  the  other.  Hence 
there  may  be  pathological  growths  of  a  character  not  normally  present 
in  the  body.  It  seems  to  me,  so  far  as  my  knowledge  of  this  subject 
enables  me  to  judge,  to  be  true  that  all  such  pathological  growths  de- 
pend ujDon  the  presence  of  comparatively  young  and  undifferentiated 
cells  being  turned  into  a  new  direction.  The  problem  of  normal 
development  and  of  abnormal  structure  is  one  and  the  same.  Both  the 
embryologist  and  the  anatomist,  on  the  one  hand,  and  the  pathologist 
and  the  clinician  on  the  other,  deal  ever  with  these  questions  of  differ- 
entiation, and  practically  with  no  others.  All  that  occurs  in  the 
body  is  the  result  of  various  differentiations,  and  whether  we  call  the 
state  of  that  body  normal  or  pathological  matters  little;  still  the  cause 
of  it  is  the  differentiation  of  the  parts. 

The  second  of  the  collateral  topics  which  I  should  like  briefly  to 
allude  to  is  another  branch  of  the  study  of  senescence.  The  fact  was 
first  emphasized  by  the  late  Professor  Alpheus  Hyatt  that  in  many 
animals  there  exist  parts  formed  in  an  early  stage  and  thereafter  never 
lost.  The  chambered  nautilus  is  an  animal  of  this  kind.  The  inner- 
most chamber  represents  the  youngest  shell  of  the  nautilus,  and  as  its 
age  increases,  it  forms  a  new  chamber  in  its  shell,  and  so  yet  more  and 
more  until  the  coil  is  complete.  When  we  examine  a  shell  of  that  kind 
we  see  permanently  before  us  the  various  stages,  both  young  and  old,  as 
recorded  in  shell  formation.  And  so  too  in  the  sea-urchin,  and  in 
many  of  the  common  shell-fish,  we  find  the  double  record,  of  youth  and 
old  age,  preserved  permanently.  This  has  made  it  possible  for  Pro- 
fessor Hyatt  and  for  Professor  Eobert  T.  Jackson,  who  has  adopted  a 
similar  guiding  principle,  to  bring  a  great  deal  of  new  light  into  the 
study  of  animal  changes,  and  to  attack  the  solution  of  problems  which 


AGE,    GROWTH   AND   DEATH  513 

without  the  aid  of  this  senescent  interpretation,  if  I  may  so  term  it, 
would  be  utterly  impossible.  This  is  an  enticing  subject,  and  I  wish 
I  had  both  time  and  competency  to  dwell  upon  it.  But  it  is  aside,  as 
you  see,  from  the  main  inquiries  with  which  we  have  been  occupied, 
for  our  inquiries  concern  chiefly  the  effect  of  cell-change  upon  the 
properties  of  the  body,  and  the  correlation  of  cell-change  with  age. 

A  natural  branch  of  our  topic  is,  however,  that  of  longevity,  the 
duration  of  life.  Concerning  this,  we  have  very  little  that  is  scien- 
tifically satisfactory  that  we  can  present.  We  know,  of  course,  as  a 
fundamental  principle,  that  every  animal  must  live  long  enough  to 
reproduce  its  kind.  Did  that  not  occur,  the  species  would  of  course 
become  extinct,  and  the  mere  fact  that  the  species  is  existing  proves, 
of  course,  this  simple  fact — that  life  has  lasted  long  enough  for  the 
parents  to  produce  offspring.  The  consideration  of  this  fact  has  led 
certain  naturalists  to  the  supposition  that  reproduction  is  the  cause  of 
their  termination  of  life;  but  it  is  not,  it  seems  to  me,  at  all  to  be  so 
interpreted.  We  know,  in  a  general  way,  that  large  animals  live  longer 
than  small  ones.  The  elephant  is  longer  lived  than  the  horse,  the  horse 
than  the  mouse,  the  whale  than  the  fish,  the  fish  than  the  insect,  and 
so  on  through  innumerable  other  instances.  At  first  this  seems  a 
promising  clue,  but  if  we  think  a  moment  longer  we  recognize  quickly 
the  fact  that  a  parrot,  which  is  much  smaller  than  a  dog,  may  live 
one  hundred  years,  whereas  a  dog  is  very  old  at  twenty.  There  are 
insects  which  live  for  many  years,  like  the  seventeen-year  locusts,  and 
others  which  live  but  a  single  year  or  a  fraction  even  of  one  year,  and 
yet  the  long-lived  and  the  short-lived  may  be  of  the  same  size.  It  is 
evident,  therefore,  that  size  is  not  in  itself  properly  a  measure  of  the 
length  of  life.  Another  supposition,  which  at  first  sounds  very  attract- 
ive, is  that  which  explains  the  duration  of  life  by  the  rate  of  wear,  of 
the  using  up,  of  the  wearing  out,  of  the  body.  This  theory  has  been 
particularly  put  forward  by  Professor  Weismann,  who  in  his  writings 
calls  it  the  AhnutzungstJieorie — the  theory  of  the  wearing  out  of  the 
body.  But  the  body  does  not  really  wear  out  in  that  sense.  It  goes  on 
performing  the  functions  for  a  long  time,  and  after  each  function  is 
performed  the  body  is  restored,  and  we  do  not  find  at  death  that  the 
parts  have  worn  out.  But,  as  we  have  seen,  we  do  find  at  death  that 
there  has  been  an  extensive  cytomorphosis,  cell-change,  and  that  the 
living  material,  after  having  acquired  its  differentiation,  passes  now 
in  one  part,  now  in  another,  then  in  a  third,  to  a  yet  further  stage,  that 
of  degeneration,  and  the  result  of  degeneration,  or  atrophy,  as  the  case 
may  be,  is  that  the  living  protoplasm  lofees  its  living  quality  and  be- 
comes dead  material,  and  necessarily  the  functional  activity  ceases.  We 
must,  it  seems  to  me,  conclude  that  longevity,  the  duration  of  life, 
depends  upon  the  rate  of  cytomorphosis.      If  that  cytomorphosis  ia 


514  POPULAR   SCIENCE  MONTHLY 

rapid,  the  fatal  condition  is  reached  soon;  if  it  is  slow,  the  fatal  condi- 
tion is  postponed.  And  cytoniorphosis  in  various  species  and  kinds  of 
animals  must  proceed  at  different  rates  and  at  different  speeds  at 
different  ages.  Birds  grow  up  rapidly  during  their  period  of  develop- 
ment; the  cell  change  occurs  at  a  high  speed,  far  higher  than  that 
which  occurs  in  man,  probably,  during  his  period  of  development.  But 
after  the  bird  has  acquired  its  mature  development,  it  goes  on  almost 
upon  a  level  for  a  long  time ;  the  bird  which  becomes  mature  in  a  single 
year  may  live  for  a  hundred  or  even  more.  There  can  be  during  these 
hundred  years  but  a  very  slow  rate  of  change.  But  in  a  mammal,  a 
dog  or  a  cat,  creatures  of  about  the  same  bulk  as  some  large  birds, 
we  find  that  the  early  development  is  at  a  slower  rate.  The  animals 
take  a  much  longer  period  to  pass  through  their  infancy  and  reach 
their  maturity,  but  after  they  have  reached  their  maturity  they  do  not 
sustain  themselves  so  long.  Their  later  cytomorphosis  occurs  at  a 
higher  speed  than  the  bird's.  This  is  a  field  of  study  which  we  can 
only  recognize  the  existence  of  at  present,  and  which  needs  to  be  ex- 
plored before,  to  any  general,  or  even  to  a  special  scientific,  audience, 
any  promising  hypotheses  can  be  presented.  Definite  conclusions  are 
of  course  still  more  remote. 

Next  as  regards  death.  The  body  begins  its  development  from  a 
single  cell,  the  number  of  cells  rapidly  increase,  and  they  go  on  and 
on  increasing  through  many  years.  Their  whole  succession  we  may 
appropriately  call  a  cycle.  Each  of  our  bodies  represents  a  cell  cycle. 
When  we  die,  the  cycle  of  cells  gives  out,  and,  as  I  have  explained  to 
you  in  a  previous  lecture,  the  death  which  occurs  at  the  end  of  the 
natural  period  of  life  is  the  death  which  comes  from  the  breaking 
down  of  some  essential  thing — some  essential  group  of  members  of 
this  cell  cycle;  and  then  the  cycle  is  broken  up.  But  the  death  is  the 
result  of  changes  which  have  been  going  on  through  the  successive 
generations  of  cells  making  up  this  cycle.  There  are  unicellular  organ- 
isms; these  also  die;  many  of  them,  so  far  as  we  can  now  determine, 
never  have  any  natural  death,  but  there  are  probably  others  in  which 
natural  death  may  occur.  It  is  evident  that  the  death  of  a  unicel- 
lular organism  is  comparable  to  the  death  of  one  cell  in  our  own  bodies. 
It  is  not  properly  comparable  to  the  death  of  the  whole  body,  to  the 
ending-up  of  the  cell  cycle.  Is  there  anything  like  a  cell  cycle  among 
the  lower  organisms?  among  the  protozoa,  as  the  lowest  animals  are 
called  ?  It  has  been  maintained  by  a  French  investigator,  by  the  name 
of  Maupas,  that  such  a  cycle  does  exist,  that  even  in  these  low  organisms 
there  is  a  cell  which  begins  the  development,  and  that  gradually  the  loss 
in  the  power  of  cell  multiplication  goes  on  until  the  cycle  gives  out 
and  has  to  be  renewed  by  a  rejuvenescent  process,  and  this  rejuvenating 
process  he  thinks  he  has  found  in  the  so-called  conjugating  act  of  these 
animals,  in  which  there  occurs  a  curious  migration  of  the  nucleus  of 


AGE,    GROWTH   AND   DEATH  515 

one  individual  into  the  cell  body  of  another.  Whether  he  is  right  or 
not  remains  still  to  be  determined.  You  will  recognize,  I  hope,  from 
what  I  have  said,  that  we  have  now  some  kind  of  measure  of  what  con- 
stitutes old  and  young.  We  can  observe  the  difference  in  the  propor- 
tion of  protoplasm  and  nucleus,  the  increase  or  diminution,  as  the  case 
may  be,  of  one  or  the  other.  If  it  be  true  that  there  is  among  protozoa, 
among  unicellular  animals,  anything  comparable  to  the  gradual  decline 
in  the  growth  power  which  occurs  in  us,  we  shall  expect  it  to  be 
revealed  in  the  condition  of  the  cells — to  see  in  those  cells  which  are 
old  an  increase  in  the  proportion  of  protoplasm,  and  consequently  a 
diminution  in  the  relative  amount  of  nucleus.  That  subject  is  now 
being  investigated,  and  we  shall  probably  know,  within  a  few  years  at 
least,  something  positive  in  this  direction.  At  present  we  are  reduced 
to  posing  our  question.     We  must  wait  patiently  for  the  answer. 

The  scientific  man  has  many  occasions  for  patience.  He  has  to 
make  his  investigations  rather  where  he  can  than  where  he  would  like 
to.  Certain  things  are  accessible  to  our  instruments  and  methods  of 
research  at  the  present  time,  but  other  things  are  entirely  hidden  from 
us  and  inaccessible  at  the  present.  We  are  indeed,  more  perhaps  than 
people  in  any  other  profession  of  life,  the  slaves  of  opportunity.  We 
must  do  what  we  can  in  the  way  of  research,  not  always  that  which  we 
should  like  most  to  do.  Perhaps  a  time  will  come  when  many  of  the 
questions  connected  with  the  problems  of  growing  old,  which  we  can 
now  put,  will  be  answered,  because  opportunities,  which  we  have  not 
now,  will  exist  then.  Scientific  research  offers  to  its  devotees  some 
of  the  purest  delights  which  life  can  bring.  The  investigator  is  a 
creator.  Where  there  was  nothing  he  brings  forth  something.  Out  of 
the  void  and  the  dark,  he  creates  knowledge,  and  the  knowledge  which 
he  gathers  is  not  a  precious  thing  for  himself  alone,  but  rather  a 
treasure  which  by  being  shared  grows;  if  it  is  given  away  it  loses  noth- 
ing of  its  value  to  the  first  discoverer,  but  acquires  a  different  value 
and  a  greater  usefulness  that  it  adds  to  the  total  resources  of  the  world. 
The  time  will  come,  I  hope,  when  it  will  be  generally  understood  that 
the  investigators  and  thinkers  of  the  world  are  those  upon  whom  the 
world  chiefly  depends.  I  should  like,  indeed,  to  live  to  a  time  when  it 
will  be  universally  recognized  that  the  military  man  and  the  govern- 
ment-maker are  types,  which  have  survived  from  a  previous  condition 
of  civilization,  not  ours ;  and  when  they  will  no  longer  be  looked  upon 
as  the  heroes  of  mankind.  In  that  future  time  those  persons  who 
have  really  created  our  civilization  will  receive  the  recognition  which  is 
their  due.  Let  these  thoughts  dwell  long  in  your  meditation,  because 
it  is  a  serious  problem  in  all  our  civilization  to-day  how  to  secure  due 
recognition  of  the  value  of  thought  and  how  to  encourage  it.  I  believe 
every  word  spoken  in  support  of  that  great  recognition  which  is  due 


5i6  POPULAR   SCIENCE  MONTHLY 

to  the  power  of  thought  is  a  good  word  and  will  help  forward  toward 
good  results. 

In  all  that  I  have  said;,  you  will  recognize  that  I  have  spoken  con- 
stantly of  the  condition  of  the  living  material.  If  it  is  in  the  young 
state  it  has  one  set  of  capacities.  If  it  is  differentiated,  it  has,  accord- 
ing to  the  nature  of  its  differentiation,  other  kinds  of  capacities.  We 
can  follow  the  changing  structure  with  the  microscope.  We  can  gain 
some  knowledge  of  it  by  our  present  chemical  methods.  Fragmentary 
as  that  knowledge  is,  nevertheless,  it  suffices  to  show  to  us  that  the  con- 
dition of  the  living  material  is  essential  and  determines  what  the 
living  material  can  do.  I  should  like  to  insist  for  a  moment  upon  this 
conception,  because  it  is  directly  contrary  to  a  conception  of  living 
material  which  has  been  widely  prevalent  in  recent  years,  much  de- 
fended and  popularly  presented  on  many  different  occasions.  The  other 
theory,  the  one  to  which  I  can  not  subscribe,  may  perhaps  be  most 
conveniently  designated  by  the  term — the  theory  of  life  units.  It  is 
held  by  the  defenders  of  this  faith  that  the  living  substance  contains 
particles,  very  small  in  size,  to  which  the  vital  properties  are  especially 
attached.  They  look  at  a  cell  and  find  that  it  has  water,  or  water  con- 
taining a  small  amount  of  salts  in  solution,  filling  up  spaces  between 
the  threads  of  protoplasm.  Water  is  not  alive.  They  see  in  the 
protoplasm  granules  of  one  sort  and  another,  in  plants  chlorophyll,  in 
animals  perhaps  fat  or  some  other  material.  That  is  not  living  sub- 
stance, and  so  they  go  striking  out  from  their  conception  of  the  living 
material  in  the  cell  one  after  another  of  these  component  parts  until 
they  get  down  to  something  very  small,  which  they  regard  as  the  life 
unit.  I  do  not  believe  these  life  units  exist.  It  seems  to  me  that 
all  these  dead  parts,  as  this  theory  terms  them,  are  parts  of  the  living 
cell.  They  are  factors  which  enable  the  functions  of  life  to  go  on. 
Other  conditions  are  also  there,  and  to  no  one  of  them  does  the  quality 
of  life  properly  attach  itself.  Of  life  units  there  is  an  appalling  array. 
The  most  respectable  of  them,  in  my  opinion,  are  the  life  units  which 
were  hypothetically  created  by  Charles  Darwin  in  his  theory  of  pan- 
genesis. He  assumed  that  there  were  small  particles  thrown  off  from 
different  portions  of  the  body  circulating  throughout  the  body,  gather- 
ing sometimes  in  the  germ  cells.  These  particles  he  assumed  to  take 
up  the  qualities  of  the  different  parts  of  the  body  from  which  they 
emanated,  and  by  gathering  together  in  immense  numbers  in  the 
germ  cells  they  accomplished  the  hereditary  transmission.  We  know 
now  that  this  theory  is  not  necessary,  that  it  is  not  the  correct  theory. 
But  at  the  time  that  Darwin  promulgated  it,  it  was  a  perfectly  sound 
defensible  theory,  a  theory  which  no  one  considering  fairly  the  history 
of  biological  knowledge  ought  to  criticize  unfavorably.  It  was  a  fine 
mental  achievement,  but  I  should  like  also  to  add  that  of  all  the  many 
theories  of  life  units,  this  of  Darwin's  is  the  only  one  which  seems  to 


AGE,    GROWTH   AND   DEATH  517 

me  intellectually  entirely  respectable.  Of  supposed  structural  life  units 
there  is  a  great  variety.  Besides  the  gemmules  of  Darwin,  there  were 
the  physiological  units  of  Herbert  Spencer.  Professor  Haeckel,  the 
famous  German  writer,  has  special  structural  life  units  of  his  own 
which  he  terms  plastidules;  he  gave  them  the  charming  alliterative  title 
of  perigenesis  of  the  plastidules;  the  rhythm  of  it  must  appeal  to  you 
all,  though  the  hypothesis  had  better  be  forgotten.  Then  came  Nageli, 
the  great  botanist,  who  spoke  of  the  Idioplasma-Theilchen.  Then 
Weisner,  also  a  botanist,  who  spoke  of  the  Plassomes.  Our  own  Pro- 
fessor Whitman  attributed  to  his  life  units  certain  other  essential  quali- 
ties and  called  them  idiosomes.  A  German  zoologist,  Haacke,  has 
called  them  gemmules.  Another  German  writer,  a  Leipzig  anatomist, 
Altmann,  calls  them  granuli.  Now  these  different  life  units,  of  which 
I  have  read  you  briefly  the  names,  are  not  identical  according  to  these 
authors.  Everybody  else's  life  units  are  wrong,  falsely  conceived,  and 
endued  with  qualities  which  they  do  not  combine.  There  is  a  curious 
assemblage  here  of  doxies,  and  each  writer  is  orthodox  and  all  the 
others  are  heterodox;  and  I  find  myself  viewing  them  all  from  the 
standpoint  of  my  doxy,  that  of  the  structural  quality  of  the  living 
matter,  and,  therefore,  interpreting  every  one  of  these  conceptions  as 
heterodox,  not  sound  doctrine,  but  something  to  be  rejected,  condemned 
and  fought  against.  These  theories  of  life  units  have  filled  up  many 
books.  Among  the  most  ardent  defenders  of  the  theory  of  life  units 
is  Professor  Weismann,  whose  theories  of  heredity  many  of  you  have 
heard  discussed ;  though  I  doubt  if  many  of  you,  imless  you  recall  what 
I  said  previously,  are  aware  of  the  fact  that  the  essential  part  of 
Weismann's  doctrine  was  the  discovery  of  the  theory  of  germinal  con- 
tinuity by  Professor  Nussbaum,  whose  name  is  seldom  heard  in  these 
discussions.  Weismann  has  gone  much  farther  in  the  elaboration  of 
the  conception  of  life  units  than  any  of  the  other  writers.  He  thinks 
the  smallest  of  the  life  units  are  biophores.  A  group  of  biophores 
brought  together  constitutes  another  order  of  life  units  which  he  calls 
determinants;  the  determinants  are  again  grouped  and  form  ids;  and 
the  ids  are  again  grouped  and  form  idants.  If  you  want  to  accept 
any  theory  of  life  units,  I  advise  you  to  accept  that  of  Weismann,  for 
it  offers  a  large  range  for  the  imagination,  and  has  a  much  more 
formidable  number  of  terms  than  any  other. 

I  want  to  pass  now  to  an  utterly  different  line  of  study,  the  question 
of  psychological  development.  If  it  be  true  that  the  development  is 
most  rapid  at  first,  slower  later,  we  should  expect  to  find  proof  of 
that  rate  in  the  progress  of  mental  development.  In  other  words,  wo 
should  expect  to  find  that  the  baby  developed  faster  than  the  child 
mentally,  that  the  child  developed  faster  than  the  young  man,  and  the 
young  man  faster  than  the  old.  And  do  you  not  all  instinctively  feel 
immediately  that  the  general  assertion  is  true?    In  order,  however,  that 


5i8  POPULAR   SCIENCE  MONTHLY 

you  may  more  fully  appreciate  what  I  believe  to  be  the  fact  of  mental 
development  going  on  with  diminishing  rapidity,  I  should  like  to  pic- 
ture to  you  briefly  some  of  the  things  which  the  child  achieves  during 
the  first  year  of  its  life.  When  the  child  is  born,  it  is  undoubtedly  sup- 
plied with  a  series  of  the  indispensable  physiological  functions,  all  those 
which  are  concerned  with  the  taking  in  and  utilising  of  food.  The 
organs  of  digestion,  assimilation,  circulation  and  excretion  are  all 
functionally  active  at  birth.  The  sense  organs  are  also  able  to  work. 
Sense  of  taste  and  of  smell  are  doubtfully  present.  It  is  maintained 
that  they  are  already  active,  but  they  do  not  show  themselves  except  in 
response  to  very  strong  stimulation.  Almost  the  only  additional  faculty 
which  the  child  has  is  that  of  motion,  but  the  motions  of  the  new-born 
baby  are  perfectly  irregular,  accidental,  purposeless,  except  the  motions 
which  are  connected  with  the  function  of  sucking,  upon  which  the 
child  depends  for  its  nourishment.  The  instinct  of  sucking,  the  baby 
does  have  at  birth.  It  might  be  described  as  almost  the  only  equip- 
ment beyond  the  mere  physiological  working  of  its  various  organs.  But 
at  one  month  we  find  that  this  uninformed  baby  has  made  a  series  of 
important  discoveries.  It  has  learned  that  there  are  sensations,  that 
they  are  interesting;  it  will  attend  to  them.  You  all  know  how  a 
baby  of  one  month  wall  stare;  the  eyes  will  be  fastened  upon  some 
bright  and  interesting  object.  At  the  end  of  a  month  the  baby  shows 
evidences  of  having  ideas  and  bringing  them  into  correlation,  associa- 
tion, as  one  more  correctly  expresses  it,  because  already  after  one  month, 
when  held  in  the  proper  position  in  the  arms,  it  shows  that  it  expects  to 
be  fed.  There  is,  then,  already  evidence  and  trace  of  memory.  At  two 
months  much  more  has  been  achieved.  The  baby  evidently  learns  to 
expect  things.  It  expects  to  be  fed  at  certain  times;  it  has  made  the 
great  discovery  of  the  existence  of  time.  And  it  has  made  the  discovery 
of  the  existence  of  space,  for  it  will  follow,  to  some  extent,  the  bright 
light ;  it  will  hold  its  head  in  a  certain  position  to  catch  a  sound  appar- 
ently from  one  side;  or  to  see  in  a  certain  direction.  The  sense  of 
space  and  time  in  the  baby's  mind  is,  of  course,  very  imperfect,  doubt- 
less, at  this  time,  but  those  two  non-stuff  realities  about  which  the 
metaphysicians  discuss  so  much,  the  two  realities  of  existence  which 
are  not  material,  the  baby  at  this  time  has  discovered.  Perhaps,  had 
some  great  and  wonderfully  endowed  person  existed  who  preserved  the 
memory  of  his  own  psychological  history,  of  his  development  during 
babyhood,  we  should  have  been  spared  the  gigantic  efforts  of  the  meta- 
physicians to  explain  how  the  notions  of  space  and  time  arose.  With- 
out knowing  how,  the  baby  has  acquired  them,  and  has  already  become  a 
rudimentary  metaphysician.  We  see,  also,  at  the  end  of  the  third 
month,  that  the  baby  has  made  another  remarkable  discovery.  It  has 
found  not  merely  that  its  muscles  will  contract  and  jerk  and  throw  its 
parts  about,  which  is  doubtless  earlier  a  great  delight  to  it;  but  that 


AGE,    GROWTH   AND   DEATH  519 

the  muscles  can  contract  in  such  a  way  that  the  movement  will  be 
directed ;  there  is  a  coordination  of  the  muscular  movements.  I  should 
like  to  read  to  you  just  these  three  or  four  lines  from  Miss  Shinn,  who 
has  given  perhaps  the  best  story  of  the  development  of  a  baby  which 
has  yet  been  written.  This  is  not  merely  my  opinion,  but  also  the 
opinion  of  my  psychological  colleagues  at  Cambridge  whom  I  consulted 
before  venturing  to  express  the  idea  before  you,  and  I  find  that  they 
take  the  view  that  Miss  Shinn's  book,  which  is  charmingly  written,  is 
really  done  with  such  precision  and  understanding  of  the  psychological 
problems  involved  that  it  may  fairly  be  called  the  best  of  the  books 
treating  of  the  mental  development  of  a  baby.  Miss  Shinn  says,  re- 
ferring to  the  condition  of  the  child  at  the  end  of  two  months — "  Such 
is  the  mere  life  of  vegetation  the  baby  lived  during  the  first  two  months ; 
no  grown  person  ever  experienced  such  an  expansion  of  life — such  a 
progress  from  power  to  power  in  that  length  of  time."  She  is  not 
thinking  of  senescence,  as  we  have  been  thinking  of  it,  but  she  makes 
precisely  the  assertion,  which  seems  to  me  to  be  true,  that  the  baby  in 
two  months  has  accomplished  an  amount  of  development  which  no 
adult  is  capable  of.  And  now  at  three  months  we  find  another  great 
discovery  is  made  by  the  baby,  that  it  is  possible  to  bring  the  sensations 
which  it  receives  into  combination  with  the  movements  which  it  makes. 
It  learns  to  coordinate  its  sensory  impressions  and  its  motor  responses. 
We  hardly  realize  what  a  great  role  this  adjustment,  between  what  our 
muscles  can  do  and  what  our  senses  tell  us,  plays  in  our  daily  life.  It 
is  the  fundamental  thing  in  all  our  daily  actions,  and  though  by 
habit  we  perform  it  almost  unconsciously,  it  is  a  thing  most  difficult  to 
learn.  Yet  the  baby  has  acquired  the  art,  though  he  only  gradually  gets 
to  be  perfect  in  it.  Again  we  see,  at  the  end  of  the  fourth  month, 
that  the  baby  begins  to  show  some  idea  of  another  great  principle — 
the  idea  that  it  can  do  something.  It  shows  evidence  of  having  purpose 
in  what  it  does.  Its  movements  are  no  longer  purely  accidental.  At 
four  months  we  find  yet  another  equally  astonishing  addition  to  the 
achievements  of  this  marvelous  baby.  He  makes  the  amazing  discovery 
that  the  two  sides  of  an  object  are  not  separate  things,  but  are  parts  of 
the  same.  When  a  face,  for  instance,  disappears  by  a  person's  turning 
around,  that  face,  to  a  baby  of  one  month,  probably  simply  vanishes, 
ceases  to  exist :  but  the  baby  at  four  months  realizes  that  the  face  and 
the  back  of  the  head  belong  to  the  same  object.  He  has  acquired  the 
idea  of  objects  existing  in  the  world  around  him.  That  is  an  enormous 
achievement,  for  this  little  baby  has  no  instructor;  he  is  finding  out 
these  things  by  his  own  unaided  efforts.  Then  at  five  months  begins 
the  age  of  handling,  when  the  baby  feels  of  everything.  It  feels  urgently 
of  all  the  objects  which  it  can  get  hold  of  and  perhaps  most  of  all  of  its 
own  body.  It  is  finding  that  it  can  touch  its  various  parts  and  that  when 
its  hands  and  parts  of  its  own  body  come  in  contact  it  has  the  double 


520  POPULAR   SCIENCE  MONTHLY 

sensation,  and  learns  to  bring  those  together  and  thereby  is  manufactur- 
ing in  its  consciousness  the  conception  of  the  ego,  personal,  individual 
existence,  another  great  metaphysical  notion.  Descartes  has  said — 
Cogito,  ergo  sum — I  think,  therefore  I  am.  The  baby,  if  he  had  written 
in  Descartes's  place,  would  have  said — "  I  feel,  therefore  I  am."  The 
first  five  months  constitute  the  first  period  of  the  baby's  development. 
Its  powers  are  formed,  and  the  foundations  of  knowledge  have  been  laid. 
The  second  period  is  a  period  of  amazing  research,  constant,  uninter- 
rupted, untiring;  renewed  the  instant  the  baby  wakes  up,  and  kept  up 
until  sleep  again  overtakes  it.  In  the  six  months'  baby  we  find  already 
the  notion  of  cause  and  effect.  You  see  he  is  dealing  mostly  in  meta- 
physical things,  getting  the  fundamental  concepts.  That  there  is  such 
an  idea  as  cause  and  effect  in  the  baby's  mind  is  clearly  shown  by  the 
progress  of  its  adaptive  intelligence.  It  evidently  has  now  distinct 
purposes  of  its  own.  It  shows  clearly  at  this  age  also  another  thing 
which  plays  a  constant  and  important  role  in  our  daily  life.  It  has  the 
consciousness  of  the  possibilities  of  human  intercourse ;  it  wants  human 
companionship.  And  with  that  the  baby's  equipment  to  start  upon  life 
is  pretty  well  established.  It  has  discovered  the  material  universe  in 
which  it  lives,  the  succession  of  time,  the  nature  of  space,  cause  and 
effect,  its  own  existence,  its  ego  and  its  relationship  with  other  in- 
dividuals of  its  own  species.  Do  we  get  at  any  time  in  our  life  much 
beyond  this?  Not  very  much;  we  always  use  these  things,  which  we 
learn  in  the  first  six  months,  as  the  foundation  of  all  our  thought. 
By  eight  months  baby  is  upon  the  full  career  of  experiment  and  ob- 
servation. Everything  with  which  the  baby  comes  in  contact  interests 
him.  He  looks  at  it,  he  seizes  hold  of  it,  tries  to  pull  it  to  pieces, 
studies  its  texture,  its  tensile  strength,  and  every  other  quality  it  pos- 
sesses. ISTot  satisfied  with  that,  he  will  turn  and  apply  his  tongue ^to  it, 
putting  it  in  his  mouth  for  the  purpose  of  finding  out  if  it  has  any 
taste.  In  doing  this,  hour  after  hour,  with  unceasing  zeal,  never  inter- 
rupted diligence,  he  rapidly  gets  acquainted  with  the  world  in  which 
he  is  placed.  At  the  same  time  he  is  making  further  experiments  with 
his  own  body.  He  begins  to  tumble  about;  perhaps  learns  that  it  is 
possible  to  get  from  one  place  to  another  by  rolling  or  creeping,  and 
slowly  he  discovers  the  possibility  of  locomotion,  which  you  know  by 
the  end  of  the  year  will  have  so  far  perfected  itself  that  usually  at 
twelve  months  the  baby  can  walk.  During  this  period  of  from  five 
months  to  twelve  the  baby  is  engaged  upon  a  career  of  original  research, 
unaided  much  by  anybody  else,  getting  doubtless  a  little  help  and,  of 
course,  a  great  deal  of  protection,  but  really  working  chiefly  by  himself. 
How  wonderful  it  all  is !  Is  any  one  of  us  capable  of  beginning  at  the 
moment  we  wake  to  carry  on  a  new  line  of  thought,  a  new  series  of 
studies,  and  to  keep  it  up  full  swing,  with  unabated  pace,  all  day  long 
till  we  drop  asleep  ?     Every  baby  does  that  every  day. 


AGE,    GROWTH   AND   DEATH  521 

When  we  turn  to  the  child  who  goes  to  school,  behold  how  much 
that  child  has  lost.  It  has  difficulties  with  learning  the  alphabet.  It 
struggles  slowly  through  the  Latin  grammar,  painfully  with  the  subject 
of  geometry,  and  the  older  it  gets,  the  more  difficult  becomes  the 
achievement  of  its  study.  The  power  of  rapid  learning,  which  the 
baby  has,  is  clearly  already  lessened. 

The  introduction  of  athletics  affords  a  striking  illustration  of  the 
decline  of  the  learning  power  with  the  progressing  years.  When  golf 
first  came  in  it  was  considered  an  excellent  game  for  the  middle-aged; 
and  you  have  all  watched  the  middle-aged  man  play.  He  was  so  awk- 
ward, he  could  not  do  it.  Day  after  day  the  man  of  forty,  fifty,  or 
even  older,  would  go  to  the  golf  field,  hoping  each  time  to  acquire  a 
sure  stroke,  but  never  really  acquiring  it.  The  young  man  learned 
better,  but  the  good  golf  players  are  those  who  begin  as  children,  twelve 
and  fourteen  years  of  age,  who  in  a  few  months  become  as  expert  and 
sure  as  their  fathers  wished  to  become,  but  could  not.  In  bicycling  it 
was  the  same.  Eight  lessons  was  considered  the  number  necessary  to 
teach  the  intelligent  adult  to  ride  a  wheel.  Three  for  a  child  of  eight. 
And  an  indefinite  number  of  lessons,  ending  in  failure,  for  a  person  at 
seventy.  It  would  have  been  scientifically  interesting  to  have  kept  an 
exact  record  of  the  period  of  time  which  it  took  at  each  age  to  learn 
bicycling,  but  I  think  enough  has  been  said  to  convince  you  that  if  we 
could  acquire  such  a  measure  of  psychological  development  as  would 
enable  us  to  express  its  rate  in  figures,  we  should  be  able  to  construct  a 
curve  like  the  curve  which  I  showed  you  in  the  third  lecture  illustrating 
the  decline  in  the  rate  of  growth,  and  we  should  see  that  during  the 
early  years  of  life,  the  decline  in  the  power  of  learning  was  extremely 
rapid,  during  childhood  less  rapid,  during  old  age  very  slow.  But  the 
great  part  of  the  decline  would  occur  during  early  years. 

Here  we  see  the  principle  of  stability,  in  maturity,  which  we  see 
also  illustrated  in  structure  and  growth.  The  mind  acquires  its  devel- 
opment; it  retains  that  development  in  the  adult  a  long  time.  But 
surely  there  comes  a  period  when  the  exercise  of  the  mind  is  difficult. 
It  requires  a  great  effort  to  do  something  new  and  unaccustomed.  A 
sense  of  fatigue  overwhelms  us.  I  believe  that  this  principle  of  ps)'- 
chological  development,  paralleling  the  career  of  physical  development, 
needs  to  be  more  considered  in  arranging  our  educational  plans.  For 
if  it  be  true  that  the  decline  in  the  power  of  learning  is  most  rapid  at 
first,  it  is  evident  that  we  want  to  make  as  much  use  of  the  early  years 
as  possible — that  the  tendency,  for  instance,  which  has  existed  in  many 
of  our  universities,  to  postpone  the  period  of  entrance  into  college,  is 
biologically  an  erroneous  tendency.  It  would  be  better  to  have  the 
young  man  get  to  college  earlier,  graduate  earlier,  get  into  practical 
life  or  into  the  professional  schools  earlier,  while  the  power  of  learning 
is  greater. 


522  POPULAR   SCIENCE  MONTHLY 

Do  we  not  see^  in  fact,  that  the  new  ideas  are  indeed  for  the  most 
part  the  ideas  of  young  people.  As  Dr.  Osier,  in  that  much-discussed 
remark  of  his,  has  said,  the  man  of  forty  years  is  seldom  the  productive 
man.  Dr.  Osier  also  mentioned  the  amiable  suggestion  of  Trollope  in 
regard  to  men  of  sixty,  which  has  been  so  extremely  misrepresented  in 
the  newspaper  discussions  throughout  the  country,  causing  biologists 
much  amusement.  But  I  think  that  Dr.  Osier  probably  took  a  far  too 
amiable  view  of  mankind,  and  that  in  reality  the  period  when  the  learn- 
ing power  is  nearly  obliterated  is  reached  in  most  individuals  very 
much  earlier.  As  in  every  class  of  biological  facts,  there  is  here  the 
principle  of  variation  to  be  kept  in  mind.  Men  are  not  alike.  The 
great  majority  of  men  lose  the  power  of  learning,  doubtless  some  more 
and  some  less,  we  will  say,  at  twenty-five  years.  Few  men  after  twenty- 
five  are  able  to  learn  much.  They  become  day  laborers,  mechanics, 
clerks  of  a  mechanical  order.  Others  probably  can  go  on  somewhat 
longer,  and  obtain  higher  positions;  and  there  are  men  who,  with  ex- 
treme variations  in  endowment,  preserve  the  power  of  active  and  orig- 
inal thought  far  on  into  life.  These  of  course  are  the  exceptional 
men,  the  great  men. 

We  have  lingered  so  long  together  studying  phenomena  of  growth, 
that  it  is  natural  to  allude  to  one  more,  which  is  as  singular  as  it  is 
interesting,  namely,  the  increase  in  size  of  Americans.  It  was  first 
demonstrated  by  Dr.  Benjamin  A.  Gould  in  his  volume  of  statistics 
derived  from  the  records  of  the  Sanitary  Commission — a  volume  which 
still  remains  the  classic  and  model  of  anthropometric  research.  Any 
one,  however,  can  observe  that  the  younger  generation  of  to-day  tends 
conspicuously  to  surpass  its  parents  in  stature  and  physical  develop- 
ment. How  to  explain  the  remarkable  improvement  we  do  not  know. 
Our  discovery  of  the  fact  that  the  very  earliest  growth  is  so  enormously 
rapid,  makes  that  earliest  period  especially  important.  If  the  initial 
growth  can  be  favored  a  better  subsequent  development  presumably 
would  result.  In  brief,  I  find  myself  led  to  the  hypothesis  that  the 
better  health  of  the  mothers  secures  improved  nourishment  in  the  early 
stages  of  the  offspring,  and  that  the  maternal  vigor  is  at  least  one 
important  immediate  cause  of  the  physical  betterment  of  the  children. 
Much  is  said  about  the  degeneracy  of  the  American  race,  but  the  con- 
trary is  true — ^the  American  race  surpasses  its  European  congeners  in 
physical  development. 

You  will  naturally  wish  to  ask,  before  I  close  the  series  of  lectures, 
two  questions.  One,  how  can  rejuvenation  be  improved ;  the  other,  how 
can  senescence  be  delayed.  These  questions  will  strike  every  one  as 
very  practical.  But  the  first,  I  fear,  is  not  an  immediately  practical 
question,  but  rather  of  scientific  interest,  for  we  must  admit  that  the 
production  of  young  individuals  is,  on  the  whole,  very  well  accom- 
plished and  much  to  our  satisfaction.     But  in  regard  to  growing  old. 


AGE,    GROWTH   AND   DEATH  523 

in  regard  to  senescence,  the  matter  is  very  different.  There  we  should, 
indeed,  like  to  have  some  principle  given  to  us  which  would  delay  the 
rate  of  senescence  and  leave  us  for  a  longer  period  the  enjoyment  of 
our  mature  faculties.  I  can,  as  you  have  readily  surmised  by  what  I 
have  said  to  you,  present  to  you  no  new  rule  by  which  this  can  be  ac- 
complished, but  I  can  venture  to  suggest  to  you  that  in  the  future 
deeper  insight  into  these  mysteries  probably  awaits  us,  and  that  there 
may  indeed  come  a  time  when  we  can  somewhat  regulate  these  matters. 
If  it  be  true  that  the  growing  old  depends  upon  the  increase  of  the 
protoplasm,  and  the  proportional  diminution  of  the  nucleus,  we  can 
perhaps  in  the  future  find  some  means  by  which  the  activity  of  the  nuclei 
can  be  increased  and  the  younger  system  of  organization  thereby  pro- 
longed. That  is  only  a  dream  of  the  possible  future.  It  would  not 
be  safe  even  to  call  it  a  prophecy.  But  stranger  things  and  more 
unexpected  have  happened,  and  perhaps  this  will  also. 

I  do  not  wish  to  close  without  one  added  word.  The  views  which 
I  have  presented  before  you  in  this  series  of  lectures  I  am  personally 
chiefly  responsible  for.  Science  consists  in  the  discovery  made  by  indi- 
viduals, afterwards  confirmed  and  correlated  by  others,  so  that  they 
lose  their  personal  character.  The  views  which  I  have  presented  to 
you,  you  ought  to  know  are  still  largely  in  the  personal  stage.  Whether 
my  colleagues  will  think  that  the  body  of  conceptions  which  I  have 
presented  are  fully  justified  or  not,  I  can  not  venture  to  say.  I  have 
to  thank  you  much,  because  between  the  lecturer  and  his  audience  there 
is  established  a  personal  relation,  and  I  feel  very  much  the  compliment 
of  your  presence  throughout  this  series  of  lectures,  and  of  the  very 
courteous  attention  which  you  have  given  me. 

To  recapitulate — for  we  have  now  arrived  at  the  end  of  our  hour — 
we  may  say  that  we  have  established,  if  my  arguments  before  you  be 
correct,  the  following  four  laws  of  age. 

First,  rejuvenation  depends  on  the  increase  of  the  nuclei. 

Second,  senescence  depends  on  the  increase  of  the  protoplasm,  and 
on  the  differentiation  of  the  cells. 

Third,  the  rate  of  growth  depends  on  the  degree  of  senescence. 

Fourth,  senescence  is  at  its  maximum  in  the  very  young  stages,  and 
the  rate  of  senescence  diminishes  with  age. 

As  the  corollary  from  these,  we  have  this — natural  death  is  the  con- 
sequence of  cellular  differentiation. 


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